Help diagnose this hardware problem please!

Hi all.

My old PC started playing up two weeks ago after we had a storm and the house electrics got knocked out - it could have been a power surge.
Basically when I start the PC nothing shows up on the monitor, the keyboard lights don't come on and the cd drive doesn't open.

I immediately suspected it was the PSU and just replaced it with a new one but it hasn't fixed it. The PSU was fairly old so it needed replacing anyway but I felt sure it was the problem.

What else could be causing this?
I'm now suspecting the motherboard but don't really want to go and buy one until I am pretty damn sure.

The PC specs;
Pentium 4 (S478) 2.6Ghz Northwood CPU
Asus P4P800SE motherboard
Corsair 1GB PC3200 RAM
Corsair 400W PSU (new one)
Maxtor 120GB HDD

Any help would be appreciated.