possible causes:
1. startup items (certain applications is running on startup and become corrupted.)
2. virus. (created a copy on system startup)
3. driver problem.
go into safe mode. then type in run command msconfig. uncheck all startups first or retain your anti-virus. reboot. if the problem still persists, do a system restore.
if the system restore option does not change the behavior, use a windows system repair.
feedback for the results.
alternate steps:
1. run hijackthis application. post the log here.
2. use combofix.