Hi, I cannot see anything obviously wrong in the HijackThis! log.

As well as what has already been suggested:

Sometimes the BIOS configuration changes the boot device priorities and the systems date/time automagically.
If you have a multimeter, test the CMOS battery. Otherwise just replace it, they are not expensive.

Use this free software to test your RAM:

http://www.memtest86.com/

Just download the free tool. The prices that you see are for the CD version

Try running your AV/malware scans in safe mode. That way only one tool at a time will be running.

Use the Event Viewer to see if there are any messages in the system logs.