7 Crashes - Can't Figure It Out

Having a really strange issue with one of my 7 systems.

When being used the system simply freezes. Ctrl+Alt+Delete does nothing, no HDD activity, just fans on, system frozen.

Usually happens when I'm working in Excel 2007.

I just replaced the Video Card of this machine with a brand new one (initially thought it was a video card and/or driver). Still crashes once in a while.

Specs:

Intel Core i3
Gigabyte Motherboard
4GB DDR3 RAM
EVGA 9500GT Card

Latest video card drivers, latest motherboard bios drivers, Windows 7 updates, etc.

Two questions:

1) What could be causing this crash?
2) Are there any logs I can look at to see exactly what happened before the freeze (and where can I find them)

Thanks in advance for the tips and helps

Thanks Jonathan,

I'll try Driver Verifier

To clarify: I'm not getting a BSOD, I'm just getting a screen freeze - Excel or the program I'm using simply freezes, nothing works (mouse, key commands, reset button on the tower, etc. must do a hard power reset only)

Jonathan,

I ran the Driver Verifier, nothing.

I'm suspecting either bad RAM, a virus (although not likely)...

What are some good Memory RAM checkers?

My next steps are:

1) Test the RAM
2) Do a deep anti-virus scan
3) not sure....