RC dog slow on Dell Optiplex GX520

Normal install (32-bit)
Standard GX520, no addon cards.
I haven't installed any new drivers/apps. I literally just clean installed the RC (build 7100) over a copy of XP Pro (the usual windows dual boot menu at start) and XP is still fine but when I log into win 7 it's slow as can be. incredibly slow is how I would describe it. Anyway the weird thing is the CPU usage and memory usage are low so I can't figure it out. maybe it's disk usage but I can't even run any portable diagnostic software because it's so damn slow. Is there anything I can check. it must be something simple. And I'm sure somebody else has tried it on a GX520 business machine. Any help is much appreciated.

It is a pentium 4.. perhaps you should just stick with XP.

How much RAM?

Er,

It's actually a pentium D @ 2.8GHz with 1GB of ram.
and it should be more than able to handle Windows 7, at least according to

Lifehacker - Windows 7 System Requirements Less Hungry than Vista's Were - Windows

I mean, when I say it's slow, I mean it's slower than vista would run on a 286... lol

I believe there's something else wrong like a dodgy driver or something else, something simple. I'm not sure what but maybe somebody with more experience of the OS could give me some tips?