no need to reformat. just run msconfig and loadup the software needed. then run a scan and delete. run spyware etc. this will take about 1 day depending on the size of the disc.
i also recommend HijackThis, its the most effective app ive used. it is risky editing the registry though, i suggest doing a system scan and saving a log file prior to the system getting infected so that you have a reference guide of what actually belongs there. with that said, my system recently got infected. i ended up reformatting the drive and reinstalling everything, HijackThis wouldve work except that the registry wouldnt delete. plus my system was due for a spring cleaning.