The first things to do for a PC that's been sitting for so long are to:
1. Replace CMOS battery (you already figured that one out)
2. Remove and reseat all cables and power connectors, and
3. Remove memory and PCI cards, clean contacts (red pencil eraser works) and reinstall
Disconnect the hard disk. Boot from a bootable CD. If you can't get optical drive operable, replace it (borrow one from another PC). It's possible removing and reseating power cables may fix the problem.
The UBCD is a good boot disk for testing PCs.
http://www.ultimatebootcd.com/download.html