We actually noticed this at our practice as well with 6.1.2, we've decided for $139.00 you can get a OEM copy of windows 7 and install it on the computer. This obviously fixed the issue but I am still unsure why XP was slow.

Server: HP DL160 G6 - 2.0GHz Quad Core Xeon, 12GB RAM, 250GB 7200RPM in RAID1.

Workstation: Lenovo 1.86 Core2Duo, 2GB RAM, 160GB Hard Drive (XP) upgraded to Windows 7.