Well my first suggestion would be to check for a virus. It is very possible your main account is infected but your guest account has yet to be compromised.
Download TDSSKiller and RKill from our
Freeware DB. If you do not have an anti-virus then download, install, and allow to update Malwarebytes Anti-Malware (MBAM). Then reboot your system into safe mode without networking (Reboot and, when it says "Starting Windows," Press F8; If you get a message asking continue in safe mode or perform a system restore, choose continue in safe mode). Now run TDSSKiller and after that run RKill and after that run a full virus scan. The first two will take maybe 5 minutes each but a full virus scan can take anywhere from 30 minutes to several hours depending on your hard drive size and number of files.
TDSSKiller is designed specifically to look for rootkits. RKill is designed to stop the stealthing programs that hide/protect/regenerate some nasty viruses. Running these first will help ensure your A/V has the absolute best chance of cleaning up infected files.
...if your system is clean,
then you might try running
CCleaner, the first section to free up hard drive space and remove temp files as well as the second part to clean the registry.