I have an 8 year old heavily upgrade HP Pavilion desktop 762C running dual boot Win XP Pro (installed first) and Win 7 Pro. I mainly use Win 7. Sometimes when booting, especially after Windows Updates are done, I reboot and get the "error loading os" message. If I put something in my first DVD drive (install or repair disc) I get prompted to hit enter to boot to the dvd. If I ignore it Window usually boots normally (but does not give me my boot menu). If I go into repair, REPAIR START never finds any error but after that boots work normally for a while.
I have an even stranger keyboard problem that started a few weeks ago - I lose keystrokes. My system is clean of malware, etc. Running NIS 2010. I followed a suggestion I got to uninstall keyboards, next boot keyboard worked normally for a while. Then not. I've gone through a few known good kbs, both PS2 and USB. Still no solution.
Hardware wise - P4 3.06 hyperhread, upgraded phoenix bios, upgraded ATI 26meg ram video plus a SB Audigy cards, Maxtor 200g drive primary and a Seagate 250gig secondary, both are PATA. Also a couple USB based drives. 2GB ram. 2gb flash drive added on a USB2 slot.