I tried to install extra memory (SIMM) on my old Compaq Deskpro 486/25, but when I restart, there's an error message " Base Memory Error 201"...Apparently, it doesnt like the new memory module. So, I took it out and restarted again. Now, the PC wont boot, but gives error "Non-System Disk error ...replace and strike any key when ready"...I think it's asking for a bootable disk on the floppy drive A. Why wont it boot from the C drive, as before? Did I changed something to the boot sequence without realising it? Please help. Thanks.

make sure that the cables on the hd did not work loose or any cable

I think with Compaq computers you have to use a setup disk in order to access your bios. Go into bios and check that your memory is reading the correct amount. Compaq doesn't like all memory modules, but if you do install generic make sure to put them in in pairs. If you've got memory that came with your computer use that in the first two banks and use the addittional generic memory in the last two banks. If you've got onboard memory and only three extra banks, use the onboard and the first bank as Compaq memory and the other two as your memory upgrade. I think you can still get Bios setup disk from Compaq's web site.