I see many causes for that on the web. You already tried the usual so keeping in mind the cost of repair today I spotted this on Woot: http://computers.woot.com/?ref=gh_cp_3
That's 140 (plus 5 for shipping) and in my town the shop minimum charge is about 125.
Again, you tried the usual reseating of parts. One last idea is to replace the CMOS BIOS BATTERY. That's about 2 dollars ( https://www.google.com/#q=cr2032 ) and I have to write no, it's not a sure thing but I find many forget to replace that part.
I have a problem with my pc.It wont turn on when i plug it on electrical outlet.When i press the start button i can here just three "beep beep beep" sound.I tried unpluging all the cables inside and plug them back again,also ram mermories but same thing again.Can you help me about this?
HP small form factor dc7700.
CPU entium D 2.8Ghz