Isass.exe can be disguised as a virus;run your anti virus in safemode.
Alternatively,try a repair if you have a windows disk click here
If no disk,you may have a recovery/restore partition,we will need to know the make and model of the computer.
How old is the computer ?
Did you install windows ?
From safemode,check to see if you can see a recovery partition.
Start / right click my computer / manage / storage / disk management / post back what is listed.