Do everything Jeff said and here's some more info as well, borrowed from one of the regular responders in Ebay's Answer Center:
Your account has been compromised, and/or
malicious code installed on your PC without your knowledge.
Secure Your Account
Note: The live help link on this page is different than the one on the home page.
This live help is set up specifically to help deal with hijacked accounts.
Protect your identity
Clean up your computer
. Check Windows Updates ... on your Start button. You will be taken to the Microsoft site.
. Once at the Microsoft site, choose the little "Scan for updates" link ... this will take a while ...
. When the page finally loads, download EVERYTHING listed as a "Critical Update"
. Delete ALL your Temporary Internet files
. Delete ALL your cookies
. Delete ALL your Internet History files
. Make sure that your Anti-Virus software is totally current ... if it isn't ... update it.
. If your computer has a "System Restore" feature, as with Windows ME and XP, you may want to turn if OFF, for now. Anti-Virus software is NOT allowed access to the System Restore files, so, if your system were infected when last backed up, the infection may re-surface from there. If you're confident your system was not corrupt when last backed up, you may leave the System Restore feature ON.
. Restart your computer into "SAFE" mode. Your screen will look "fat and ugly" ... that's okay. Your system help should have instructions for booting into SAFE mode. On many machines, pressing the F8 button, just after the initial "black screen", but BEFORE the "starting Windows" screen appears will allow you to boot into Safe Mode.
. Run a full-system Virus scan ... scan EVERYTHING! Your scan may be slower, in Safe mode, but it will be very thorough.
. If the scan shows something abnormal, follow your Anti-Virus software's instructions for the removal of the malicious code ... then ... run your scan again. Continue running scans till a full scan shows nothing.
. Restart your computer normally, and change your password... again.
. If turned off earlier, turn your "System Restore" feature back ON