Help! Hanging PC

I recently started working for a printing company as their web designer.

Two days before I went and installed the software I require and carried out virus checks and spyware checks etc...

I discovered there was spyware on the system that was allocated to me, and had it cleaned off by using Ad-Aware 2007 Free Version, and McAfee.

Thinking the problems had been sorted I carried on using the computer as normal, I have been having problems with the PC hanging for the past 3 weeks now and is gradually getting worse day by day.

So I installed Spybot Search & Destroy which found things that Ad-aware had not found on a daily run.

So I let it clean the PC. after reboot everything seemed fine, till I ran Dreamweaver and Photoshop together and it started hanging again.

This would happen no matter which software I was using and even when I was just using Notepad

And now the PC will not load Windows Desktop at all, it freezes at the "welcome" screen.

I have been told, that it could be the motherboard, the PSU has been changed so we can rule that one out. The memory is fine also.

I am really stuck now, has anyone had the hanging problem?

When the PC was working fine after Spybot, the problem with My Documents popping up after reboot was gone, but as soon as it the PC hung up it came back.

Another issue with this PC is that I cannot get into safe mode, it reboots after loading MUS.sys or something.

Nobody in the office knows the administrator password so I cant even use the XP CD to do a FIXBOOT or FIXMBR.

I'm pulling out my hair, its costing me money :'(