I recommend you purge your system of clutter using
Windows [
XP /
Vista]
Disk Cleanup,
ATF Cleaner or
CCleaner. If you use CCleaner, then during installation, uncheck the option to install the Yahoo toolbar and before first use, go to
Options >
Settings >
Advanced and ensure
Only delete files in Windows Temp folders older than 48 hours is
unchecked.
Note: Ensure you know your site credentials (user name and password) for sites you frequent before cleaning; you may have to login again at next visit.
Then download, install, update, and run
Malwarebytes's Anti-Malware (MBAM) to ensure your system is free of malware. Then do the same for all other computers on your network (everything on your side of the Internet gateway, typically a cable/DSL modem).
If you still have problems after that, I recommend you post a HJT log following these
HijackThis Procedures.