I wasted beaucoup time over the last month with my XP Professional explorer “hanging” for minutes at a time. This happened in two main situations: when hitting the start menu key, and when right clicking on a folder. A google post finally tipped me off to the problem - the PGP service. Deinstalling PGP fixed the problem immediatly. Note sure if it was a free version I was using, but I saw on deinstall that the version number was 8.0.3.
NOTE: Upgrading to 8.1 fixed the problem. There are rumors (even though the source code for the 8.x version is published) that there are back doors in PGP for the government, but this appears to be only rumors. Phil Zimmerman (creator of PGP) says that all versions prior to 7.03 were free of back doors, but he does not know about later versions (he left the company).