My PC (Windows XP SP3, Intel Core Quad 2.66GHz, 4GB RAM) was extremely slow when Visual Studio 2010 was running. The UI seemed to be very irresponsive; every type took 10 seconds.
As this blog post shows, this is caused by an old version of Windows Automation API. Basically, VS 2010 Intellisense tries to pop up list boxes with many many entries. When any other application using Windows Automation API runs, Windows Automation tries to read any list box posted to screen and this is now very slow.
The solution is to update to Windows Automation API 3.0. Microsoft will send an email with a link to download the hot fix file and a password to unzip that file. The downloaded file was 413106_ENU_i386_zip.exe for me, and it was extracted to WindowsXP-KB971513-x86-ENU.exe using the password.