Halfway through formatting & reinstalling win xp had a powercut,when the power came back on i turned the computer back on with the win xp disc still in but ive now got nothin,the lights r on but no 1 home,the tower has powered up but the monitor comes on 4a second each time i turn the tower on but keeps putting itself in2 standby mode & wont come back on
I got to the stage where it was copying files,about 25'/.

Try removing your mother board battery for 5 mins then replace and try again.

Hi bob
cheers 4 ur advice,but didnt do anythin,any1 no anythin else i can try?

I think what you may be looking at is a minor disastor. Or a big one... stuff is likely destroyed by a power spike which got through your PS.
Swap in a working monitor.
Same no-go? Then BIOS is likely not running. If you have a video card, remove it so as to use on-board graphics. Leave only one stick of RAM, unplug any other unnecessary devices, like even your CD/DVD ROM, all other drives [yep, even your main hd], other cards.
Try again... swap RAM sticks,... nothing? Okay, now it is a big disaster. When all you have plugged is a PS, mb with RAM and a monitor, and you don't see BIOS run, what you need is a shop.
Good luck with it.

Please let us know how you get on with Gerbill's advice as it is your only chance. There is nothing anyone here can do to help you if that all fails.

Its unlikely interrupting the windows install did anything. Sounds to me like it got fried by a power surge.