If you didn't get a CD, then Windows probably resides on the computer somewhere (in case you wanted to install software from it). However, the installation isn't easy to start. However, there is a better approach.
Click Start, Run and type
and press enter. This should scan the Windows setup for missing and corrupted files. However, any updates from Windows Update may be lost, so be sure to click here to reinstall them.
Please note that a full backup is advisory before completing this (just in case) and unless you have a firewall, your computer may be vulnerable to the Sasser virus after running SFC.
No personal files will be affected, or files added to your computer since Windows was installed. Have installation discs for other programs handy, just in case.
All other precautions (ie Power failure, alien abduction, etc) apply.