Computer resets with Power cuts not alw...

My computer resets when power goes or comes. It doesn't happens always. Power Supply comes through Two UPS one bigger with external battery with which other electrical components are also attached and then through smaller 600 Watt Hours. Computer Configuration is as :- 1. Intel P4 with D865GSA motherboard. 2. Two harddisks hitachi 80GB IDE and WD 500 GB SATA. 3. LG DVD Rom IDE 4. Front USB 5. Canon Scanner CanoScan LiDE 25 through USB (Power also USB) 6. Canon LBP3000 Printer through USB power externally. 7. CRT Monitor 8. UPS Microtech This unknown computer reset often destroy most of work currently doing on PC. Please provide a solution.