Welcome to BC vegassounder
You should have gotten reinstallation disks for all the originally installed software when you bought the computer. See if the original owner or the dealer has them and GET them from him/her if you don't already have them. Also look for a greenish tag on the disk tower. This tag has the product key on it, a very important number.
If you get the run-around about the disks and if the product key is missing from the disk tower, Microsoft does have a program for those who unknowingly purchased an illegal version of Windows. You can find out about this program here: http://www.microsoft.com/piracy/Reporting_FAQ.mspx