You can reinstall the system to factory state from the recovery partition. The exact procedures vary slightly from make to make. But that wipes everything on your C: partition. You have to save your data first to an external drive or another partition. After the reinstall you have to reinstall all the programs that you installed yourself and did not come with the system. You also will get all the updates from Microsoft that have been released since your PC was built - but that will happen automatically.
If you have programs where you purchased the product key (e.g. a security program), you should secure those keys too so that you have them for the reinstallation of said programs.
For the Vista reinstallation I may have the procedure. But I need to know the make of your system. Below I give you the example for a Gateway or E-machines system: (but again note that it is different for other makes)
1. Copy all your own files and folders to an external storage (Disc, USB stick even SD card - 1 GByte capacity should be ample - even 512 MB)
2. Shut the PC off
3. Turn the PC back on and keep hitting the F8 key repeatedly until the Advanced Boot Option Screen appears
4. Click "Repair Your Computer" –if you are not already on it, use the up and down arrows to maneuver to it and then press ENTER
5. Next you have to pick your country and click OK
6. On the "System Recovery Options Screen" click "Restore Application" (last option)
7. Follow the on-screen instructions
8. When prompted, click on "Full Factory Recovery"
9. Click "Next"
10. Click "Yes"
11. Go and have a cup of coffee - don't touch anything - for a while it seems like nothing is happening
12. After many minutes, recovery will complete and you click "Finish"