PC backed up by ups restarts on power cuts

Hello,
I am having this problem for more than 2 years...
My pc is a...INTEL P4 HT 3.06Ghz+gigabytemobo+80gb segate hdd+1gb ddr2 667 ram+ 450W PSU+ 600VA ups machine. I dont have any external graphics/sound card. when the main power is on the pc runs fine. but when there is a power but from the supply station, it reboots automatically even a brand new ups is attached to it. as a result any work in progress is lost! except the cpu & hdd everything inside the pc is very new. i've tried almost everything but only to get disappointed. any help/idea to solve the problem
will be highly appreciated! I AM REALLY FADE UP WITH IT FACING FOR > 2 YEARS.
thanks in advance!