You could run SFC {System File Checker} but it will find a lot of modified files if you don't have a SP2 install CD to work from and when it's finished you'll need to reinstall all the MS Patches and SP's.
Summarily you could try a repair install but if it works you'll again have to reinstall all the newer Patches and Service Packs that have become available since your install CD was made.
Or you could also try a Restore Point prior to this happening and if that fixes the system you'll still have to reinstall all the patches and Service Packs.
The easy answer is to backup all your data wipe the HDD and reinstall the OS & software as well as all the drivers for your hardware and then you can apply any Service Packs and patches that are required.
Col