My pc wont start and im a sequanced flashing load light...

Hello, Seems my pc is playing up. built it about 2-3 years ago AthlonIIx2 240 2gb ram HD4850 Some random HDD 500wat PSU Gigabyte GAM720 US3 Mother board. Looked after it cleaned dusted ect all that stuff ya know you do, if u care but now its tempermental. Sometimes it wont boot, it starts and i get the gigabyte load screen but then nothing, at the same time the load light on the case is giving of a sequance flashing (Defo not from it loading) Also this happens even after a propper load, randomly while doing almost anything from Gaming to editing and even browsing. all the hardware seems fine, i mean well.. i havnt changed anything from when it did all work fine and everything is in the right place and all sounds and looks like its working propperly..... IM STUMPED! dont have a clue whats going on, please help me !!

Mechanical components break over time.

The PSU is almost always the first thing to die. If everything was working fine for years and then everything started screwing up, that is the problem as often as not.

The hard drive could be failing as well. You may want to do a disk check on it.