Quad is half as fast as C2D - why?

I'm completely baffled.

I need to run an Excel spreadsheet that's quite large...and I need to run it all the time. I've been running it in Excel 2007/XP on my MacBook Pro 2.4 C2D/6gb RAM via Boot Camp.

I bought a Gateway SX2802-01 (C2Quad/4gb ram) to do this one thing. It's running Win7.

When I run this Excel file on my MacBook Pro, it takes 30 seconds.

When I run it on the desktop Gateway Quad, it takes a 60 seconds or longer.

What is going on here? Excel tells me that it's using all 4 processors....but why does it take so long? Is it the OS? RAM?

Do I need to move to a custom AMD or i7 system?