Hi Brigitte. I had a look on that craigslist page; there are a couple of really good bikes there...all of which would be better than your Wal-Mart bike. Some will be almost as heavy, but all are MUCH better quality frames and have better components. I don't know if any will fit you, but they're worth a look:
http://rockies.craigslist.org/bik/230813865.html The Specialized Rockhopper is a classic vintage MTB; for $100 you'll have a great durable bike, and you can do some component upgrades if/when you wish.
http://rockies.craigslist.org/bik/231225728.html For $400??? If this bike is in reasonable condition, it's a steal for that level of quality. See this page of reviews: http://www.mtbr.com/reviews/2003_har...t_121390.shtml
http://rockies.craigslist.org/bik/223505410.html Unless I'm missing something, $225 for this bike is another steal. Definitely worth checking out.
Upgradeable, too. Some reviews:
http://www.mtbr.com/reviews/XC_Hardt...t_123421.shtml
There are also a couple more older Specialized bikes on that craigslist page. They are both nice, older bikes which would be partially upgradeable.
Good luck! Hope this helps a little.
~Sherry.