Chances are your computer has been compromised. Contact your credit card companies and bank and inform them that you may have been a victim of such an attack. From a different computer, change any web passwords you use. I highly recommend utilizing something such as password safe to securely store and generate longer more complex passwords.
If you don't have a known good system, best bet is to have your system wiped and restored to factory defaults. Be weary of the files you backup up prior to this, since those may be infected. I would back them up to an external hard drive and maybe bring it by your local computer repair shop and have them scan it for viruses. Once your new system is rebuilt, patch the thing to high heaven, install a valid anti-virus program such as Norton/McAfee/ESET (non-free) or Microsoft Security Essentials (Free). Ensure your Windows firewall is running as well or the firewall of the AV if it has one.
Finally if you are running Windows XP, do not run as a user with elevated privileges. For your every day activity run as a regular user and create an admin user to isntall software or reconfigure the system as needed. Alternatively, you can do all your internet activity within a Virtual machine using Linux. You can load Virtual Box from Sun/Oracle and install Ubuntu which is a fairly user friendly system. You can make snapshots of the VM so in the event that you think you get compromised again, you can just revert back to the snapshot.
And one more thing, if you ARE still using XP, I highly recommend moving to Windows 7. It has a bit more security controls in place than XP does.