pc won’t boot up after restart….help!!

I have an HP Pavillion 734N desktop running WinXP (sp2) that’s worked beautifully for 2+ years now.
Last week I ran MS update, installed ONLY the critical updates, and let it restart. It came up once, and locked up immediately. It has never come back up. I don’t even get a post screen. I get led lights, fans come on, moniter light blinks, and that’s it. Soooo, I replaced the power supply and no go. Still the same thing.

I have tried powering on with the drives disconected and it’s no better. Any help will be appreciated, I use it for work and am dead without it.

Thanks.