Generic Laptop mobo - Nope. Never heard of em anyway.
Salvaging parts - CD/DVD Drive + hard drive + floppy possible, using 3.5"-2.5" converters which can be bought for about £3.00 each ($5 ish)
As for keyboard, cpu, video card - dont think so. CPU May be a standard cpu, ie socket 7 or something, but keyboards are connected via ribbon cable direct to mobo etc and would not work.
To be totally honest i recon u r wasting ur time. Here are some desktop prices:
CD-RW/DVD Drive = £15.00 ($25 ish)
20gig HDD = £40 ($70 ish)
Mobo & CPU = £150 approx ($280)
video card = from £15 ($30)
The rest is cheap also. Its up to you but i know i wouldnt bother!