This post is part of a series on some of the most memorable companies that have disappeared.

Ah WorldCom. Aside from its storied history as one of the world's biggest accounting frauds, I remember it as my first cell phone company. My husband bought me a WorldCom phone as a gift and it turned out to not only have terrible service, but ridiculous billing practices, and we ended up paying to get out of the contract as I recall. I remember thinking that there was something really wrong with that company and later wishing I had pursued it as an investigative story, since I was then a writer at BusinessWeek Online and WorldCom was a hot stock.

But no, I never got onto such a story. In fact, I followed WorldCom's stock with interest since I had picked it in an office stock-picking contest years earlier and felt some satisfaction at its meteoric rise through the 1990s (even though I never actually owned the shares; it was just part of a fantasy portfolio).

But here's the WorldCom history that is worth remembering now: WorldCom started as Long Distance Discount Services (LDDS) in 1983. It changed its name to WorldCom in 1995. A series of mega-mergers transformed the company, culminating in its $40 billion deal for MCI. It was rechristened MCI WorldCom in 1998, the second largest long-distance calling company. The following year, just as it announced a deal with Sprint (now Sprint Nextel (NYSE: S)) that never came to fruition, the telecom industry started a prolonged downturn.

This post is part of a series on some of the most memorable companies that have disappeared.

Arthur Andersen (1913 to 2002) spent decades as a leading accounting and consulting firm. Founded in 1913, it was once a member of the "Big 8" accounting firms, which later became the "Big 5." Andersen was the accountant for MCI and Worldcom. Even though it's been dead for six years, it left one offspring -- it spun off its Andersen Consulting unit in 1989. The renamed Accenture (NYSE: ACN) went public at $14.50 in July 2001 -- the share price is up 176% since then.

Andersen's downfall was its role as Enron's auditor. It used its credibility to bless Enron's special purpose entities and a whole host of illegal accounting. In 2002, the firm voluntarily surrendered its licenses to practice as CPAs after being found guilty of criminal charges, resulting in the loss of 85,000 jobs.

The lesson is to resist the lure of big money to pull you away from your values. Enron's pile of cash was irresistible to Andersen's leaders. And their lack of moral fiber cost a storied and proud firm its existence.

Peter Cohan is President of Peter S. Cohan & Associates. He also teaches management at Babson College and edits The Cohan Letter. He has no financial interest in Accenture.

Let us know in the comments what you remember about Arthur Andersen. And be sure to check out other Companies That Have Vanished.

"Investors have shied away from the big telcos in recent years because of concerns that their traditional businesses were shrinking," notes George Putnam III, an expert in uncovering turnarounds.

But now, he explains, "After years of concern about the cable companies invading their turf, the big telecoms are now well positioned to fight back."

In his The Turnaround Letter, the advisor looks at seven leaders in the global telecom space, all of which he says represent global leaders, with dominant positions in their local markets and the "potential to grow steadily by expanding the services they offer."

AT&T (NYSE: T) Putnam notes, gained control of Cingular Wireless due to its merger with Bellsouth. The renamed AT&T Wireless, he says, will account for about 35% of AT&T's revenues.

The advisor observes, "In addition to a strong wireless presence, AT&T is rolling out fiber-based landline services. With revenues expected to be north of $120 billion in 2007 and substantial operating cash flow, AT&T is a force to be reckoned with." Further, he notes, the dividend was just raised for the 22nd consecutive year, and the company is expected to repurchase roughly $7 billion worth of stock in 2007.

Ted Allrich is the founder of The Online Investor and author of the just released book: Comfort Zone Investing: Build Wealth And Sleep Well At Night. In this weekly column, he'll offer advice to investors who are just getting started.

Rule no. 1: Always buy stocks with earnings. Earnings are what investors get to keep. The more earnings a stock has, the higher the price will go. Don't buy hope or future earnings. Buy earnings that are happening now, especially the ones that are increasing every year.

Rule no. 2: Always do your research. Don't buy a tip just because a talking (or screaming) head says a stock is good. It might be good for them but not for you. Since no one will tell you when to sell, if you don't do your homework, you can't know when the stock is overvalued and should be sold. It may be overvalued when you buy it. You won't know if you don't do your homework. And you won't know if it's the right type of stock for you. For example, if you're looking for a dividend and the stock is in the early stages of biotech, then it's definitely not for you. Do your research well and know what you own. Then you'll know when a stock is cheap or rich, when to buy or when to sell.

Rule no. 3: Always follow your stocks. You can't just buy and hold anymore. While you should be a reluctant seller because you've done your research and bought strong stocks, things change, things like management, competitive environment, economic conditions, etc. Nothing stays the same, ever. Sometimes the evolution works in your favor. Sometimes it doesn't. The best companies evolve ahead of change. Look at Apple, Inc. (NASDAQ:AAPL) as an example of a company that is changing dramatically, even its the name (now it's just Apple, Inc., not Apple Computer). Don't sit and hope for the best. Follow your stocks and the financial news. Use your TV computer, newspapers, magazines. News is everywhere, not in one medium.