PC Just Locking Up!?

[SnowWar]

Hi, I need help with this PC also,

System Info:

Intel P4 2.8 Ghz HT
Gigabyte 8IG1000P-G Motherboard
512MB DDR400 Ram
Intel 82865G Onboard Display
Onboard Sound
Windows XP Pro SP2

We've got an FTP server on it and also run a proxy on it to give other people internet access (we have ADSL).
Here is the situation:

I use this pc during the day and it works fine, sometimes I leave it doing nothing for hours or most of the day, but I leave work at around 16:30 and when I get in the next morning I find the pc frozen. Its still on, but the monitor light is flickering and when I move the mouse (as I usually do during the day) to wake it up nothing works. I've also notices in the bandwith monitor that this basicly always happen between 17:00 - 18:00 and that it does actually freeze. Also, the Power Settings is set to Always On.
So why would a PC freeze like this? Any why not duing the day. I'm also 100% sure than no1 from the office is touching it.

Hope someone can help me here or point me to possible causes.

Thanks

Snowy

 

[Codesmith]

Durring the day set the clock ahead and see what happens.

Check to see if you have anything scheduled to run at night, disc defrag, backup, virus scan ect.

Also when troubleshooting its usually a good idea to

Use Windows Update frequently
Scan for spyware and viruses,
Monitor voltages and temperatures,
Run Memtest86 and Prime95
Stress test with Sandra's Burn in Wizard.