Hangs during Applying Personal Settings

I've got a network of around 500 PCs running XP Sp2/Sp3 and 2003 Servers.
Occasionally (around 15-20 in the last 4 weeks) a PC hangs at the Apply
Personal Settings stage. After waiting for 10 minutes or so, switching PC off
and on again is the only solution.

There seems to be no problem with DNS, rebooting the PC makes it work fine
again - and when the PC has hung at this stage I can still connect to it
using the Computer managerment console so it seems unlikely to be a
networking issue?

The only thing I've noticed is that out of the 15-20 PCs affected, none of
them have done it twice (yet!) and all of them are different ages,
manufacturers and on several different subnets!

Any suggestions are where to look next? I'm a bit stumped on this on.