Something conflicting somewhere. Have you tried to run your anti-virus program, then you anti-spyware program, then use disk clean and then run a program like, 'Registry First Aid' (use safe option only), and defragment your HDD, and restart.
Also make sure you do not have 2 anti-virus programs active, or using too much memory and hitting buttons to get things moving.
Lots of reasons, but eliminate spyware and viruses first, and then check for highjack. Do not visit suspicious sites as you pick up lots of nasties there. Good luck.