DELL OPTIPLEX GX280 Booting issue...

[0m3GA ARS3NAL]

Okay, I'm 17, and don't know all the hard core tech-terminology that most of you wise question-answerers do, but here is my problem.

So I was playing Counter Strike, exited the game.
I shut down all programs running in the background, and shut down my computer.
Now, when I come back to turn it on, it seems okay and everything.

I can still see it go through the DELL boot screen, and I can still get the prompt to press F2 and/or F12.
After that screen is done, I get a black screen with a flashing _ , like an input prompt.
I can not input anything, there are no beep codes, and DELL Customer Service has not helped one bit. (Like they ever have, ammirite?)

The screen sits like that forever, not ever changing from a flashing prompt.

The specs on it (From my memory...)

1.5 GB RAM
2.5 GHz processor

Video is Chipset

As is audio

All my cables are plugged in, and i have not done anything for fear of damage. (I don't think I would, I've build computers before and know my way around the inside of one a fair bit.)

Any help would be appreciated.

EDIT: It's actually an OptiPlex GX260, but that probably doesn't change much...

EDIT 2:
I just went into the setup menu and checked the POST Logs....
Looks like there was a POST Error...
All it says is "Keyboard Error" for all the day's I've tried booting it up...
But... How can that be? My F2 and F12 certainly work... Could it be... something else?

[0m3GA ARS3NAL]

Double posting bad? Oh well, I can't edit my post anymore due to the silly little 30-minute editing period that the admins think is a cool idea.

ANYWAYS.
EDIT 3
Just opened it up, took out/put back the RAM, and the processor.
Tried again, no luck.

All of my USB ports are empty, not in use, and the error lights on the back are all green.
A
B
C
D

This has happened TWICE before.
The first time was a different PC, still a DELL though. (It was an actual tower looking case, not the kind you see in schools nowadays, the kind monitors sit on usually...)
The second time, it was with the current problem computer, but I dunno how it was fixed.
We sent it to some dude, he checked it out, and all was well with the world.
Dunno what he did, and I don't know how to contact him.

ALSO, I was going to change the Processor speed from 2.5 to 2.66 GHz, cause that is what it actually was...

[W1ND0W5]

Your Windows install/MBR might be corrupted, do you have a Windows CD-ROM? And, what OS are you running?

[Abraham12]

Thank for sharing this valuable information.

[0m3GA ARS3NAL]

Sorry for the late answer, I didn't think anyone would answer since it'd been so long.

I'm on Windows XP Pro, and no, I don't have the disc, it came with the PC...
(PRIME example why companies should include a recovery disk...)

[W1ND0W5]

OK, it's okay about the slow response, um, you're gonna have to buy an XP Pro disc so you can carry out the recovery process.