My computer-C drive-and start looking------delete everything in temp, temp internet and cookie files in C-windows. Download the free version of adaware from click here to clean any advertising progs. Update your AV and run a scan. If you have XP go to control panel-folder options and tick 'show all files'. Then go to C-documents and settings-the users -local settings- and delete all in the history, temp and temp internet files.
Personally I would be tempted to do a complete reformat.