First, change your password.
Now that thats out of the way, run an (updated) virus scan, if you dont have one, heres a free one: Free Virus Scan
If that comes up clean, run a spyware / malware checker. If you do not have one, download a free one here (dont forget to update it before scanning!): Ad-aware
If that comes up clean, you should be fine now that you've changed your password. Make sure to use an alpha-numeric combination!!!