No idea

I was brought a box last night, a friend of my dad's friend needed help with their old computer. I agreed. The computer is an old Dell Dimension 4700. Pentium 4 CPU, 508MB RAM running Windows XP, but I believe the issue is hardware related.
I finished cleaning the viruses from the hard drive and such and shutdown the system, I then opened the box to clean out dust buildup which was very heavy. keep in mind the CPU has no fan attached to the heatsink. The fan is on the back of the computer and sucks air in from the back, a green plastic "tunnel" directs the air to the heatsink for cooling. I removed the fan with the "tunnel" and used compressed air to blow out the dust. I then used a vacuum cleaner with the small nozzle attachment to suck the dust out of the area the CPU was at. I then re-attach the fan to the back and plug it back in to it's spot on the motherboard.
I then bring the box into my room, plug in the power cable and VGA cable and press the power button. Now it powers on with all the fans at full speed, does 6 long beeps and then idles. Nothing comes up on the screen. Any idea what the heck happened?

EDIT:
Beep sequence is 1-3-2

EDIT 2:
Upon further Google-ing, I found the beep sequence was a RAM seating issue, which puzzles me due to the reason I never touched the RAM.

that post sequence is ram related. try reseating the ram.

Remove all the memory modules, then install only one module in slot 1, then see if the system boots and if the computer starts normally, reinstall an additional module.

Does not boot, move the module to slot 2 and again see if the system boots, no boot, try the module in slot 3, no boot, try installing in slot 4.

Still does not boot, then remove the first module and using the second one, repeat the sequence described above.

Still no boot, try installing any remaining modules, one at a time, using the above procedure.

Continue until you have identified a faulty module [or slot] or reinstalled all modules without error.

Still does not boot, buy new compatible memory module/s and try them.

There were 4 channels, with only 2 channels occupied. I reseated both Sticks in the unoccupied channels and the system booted. I should learn about beep codes now so I don't get a sinking feeling like I had there.

Lol i fix no end of computers for people, and i always get that sickening feeling when things wont post or odd things happen lol..
Good job i dont charge for my work

I'm the computer guy for my family and my dad has been spreading the word around his workplace and his friend joined into that after I fix his system. This is the first one I'm going to start charging for, it's a good way to make money for things I need and very few things I want. hahah.

im to laid back to charge.. and that way i dont have to rush lol, ive had a Dell XPS laptop sat on my table for a couple of weeks now havent actually got round to it... BF3 has taken hold!