you do not necessarily need to go off and buy anything
lets start by you telling us your windows version; what is ( or was) your installed antivirus program what other protection methods are ( or were ) on there; when were they all last fully updated and run on full computer scans?
can you still connect TO the internet? and if ON xp do you have SP2 installed and are all your windows updated up TO date?
is system restore enabled?
armed with THAT info we can maybe suggest some FREE programs you can try to start to sus out what IS going on and how to start to see if we CAN clean you