first of all, do you run any anti-virus software? if you do (mcaffee, norton, are two examples) then update them and do a full scan with them.
secondly, do you have any spyware removal software? if not, download and install Ad-Aware.
What internet browser do you use? Just standard internet explorer? If so, open it up and goto tools->internet options. click the 'delete cookies' and 'delete files' buttons