How about a Dell Optiplex GX260? Take a look at this web site
and make sure it applies to your system.
Dell OptiPlex GX260 Systems User's Guide
http://support.dell.com/support/edocs/systems/opgx260/en/ug/index.htm
If that's your computer, take a look here,
Dell OptiPlex GX260 System Setup
http://support.dell.com/support/edocs/systems/opgx260/en/ug/advfeat.htm#1101564
Scroll down to the "System Setup Options" section. "Auto Power
On" is the feature you'll need to enable. Unfortunately, here's
where the documentation gets a little vague. They don't
describe how you set this up. It's restart your computer. Hit
the F2 key as soon as you see anything displayed on the
monitor. From there you're on your own. I'd be on the lookout
for a section that has Power somewhere in the title.
Due to the sketchy instructions I'll pass along this advice.
First off, poking around in the system's BIOS can be tricky.
It's not something that the average computer user should even
consider messing around with. Before you make any changes be
certain that any important files on your computer are backed
up. Finally, ask yourself if having your computer automatically
start is worth the risk.
Note: If your computer is plugged into a power strip or surge
protector don't turn off those devices. There has to be a
source of power to the computer in order for it to start
automatically.
Good luck
Nepatsfan
(e-mail address removed),