Error warning on boot up, and more!

I’ve just got hold of a second hand PC, (Win 98se, 20mb Hard Drive, Pentium 3, 500mhz processor, 256mram)which has been working like a dream since I got it. Suddenly, however, I’m getting a warning on boot up : ‘Warning, SPD not found at DIMM(s)3’. I’ve ignored it up until now, cos the PC seemed to be working fine. Tonight though. I installed AOL 8, and suddenly most of my previously installed programs won’t even open! They don’t even appear on ‘Add/Remove programs’ in Control Panel! I’ve also got an unidentified device appearing, though I have absolutely NO idea what it is!
Any help would be greatly appreciated.