Mouse doesn't work on initial bootup

Hello,

Recently, upon initial bootup, the mouse has stopped working. It will only work after rebooting into "Safe Mode" once or twice. I tried replacing the mouse with a new one but it didn't help. I checked the BIOS settings and neither the "Auto" nor the "Enabled" setting had any favorable results. Device manager shows a mouse driver present.
I also did a clean re-installation of Win XP Pro SP2 to no avail. Virus and Malware scans turned up nothing. The computer has worked fine for six years without a hitch until a couple of weeks ago. I am at a loss as to what the cause of the problem is. Any suggestions?