what you mean with "how reinstall"?
usually (if you have installation disk and all drivers - what you really must have) - save your documents from system disk (say c:\) - all folders made by you, desktop, my documents and favorite from profile(s), boot from cd and install system from scratch formatting system disk, then drivers etc., then turn your files/folders back. sure, with this you will need to install again everything you had before, but it is best way on my opinion - i always do same at mine (already must update my system at home - when time will come

)
regards!