From my experience Windows Server 2003 Enterprise edition
without service pack 1 was much faster than WinXP PRO SP2.

For me, what I do after installing 2003 is enable 'no ctrl+alt+del startup',
and disable 'system event tracking'. Both through 'gpedit.msc'...

Above are all I do, and 32mb SPI times with 2003 server were a lot
faster than WinXP PRO SP2 for me.

Do you use any special tweaks with XP PRO SP2?
I think it might be that you're XP SP2 PRO is tweaked really good.

Otherwise think something went wrong with you're 2003 server CD.
Could it be the R2 trial edtion's specific prob? Mine is a retail version..