What is the make and model of the computer?
What OS disks do you have? An OEM recovery or a Microsoft 2K CD?
A Repair or Clean install might be the most expedient answer.
Otherwise:
As a first response you might want to post a HijackThis log for review by our Pros in that section:
Please follow the instructions
HERE & then post your log in a new thread in the
Spyware, Adware, Viruses and HijackThis Logs section.
(
Not in this section please).
Please specify what issues you appear to be experiencing along with your log.