Generally running all the time -
Firefix 3.6, typically 4 tabs including LiveJournal, FaceBook, SCA related pages, BBC news etc
Email - MailCOPA (V11)
AIM, YIM, GTalk (usually latest versions, as and when I remember)
Windows sidebar with calculator, moonphase, sticky-note, currency convertor & dictionary lookup
+ AVG virus and Spybot S&D
Applications in use (various combinations, but sometimes none)
MS Office 2007 (mostly Word & Excel)
Adobe InDesign CS3
After performing some task (reading an email, looking at a web page, some other (there does not seem to be a defined pattern), I move the mouse to the task bar to select another application. I click, but instead of bringing that application forward, I get the hourglass. While this is going, I cannot left or right-click on anything in the task bar. I can alt-tab to any of the running applications and use them without apparent problems. This can last 30 seconds, sometimes more. On a couple of occasions, I have had to invoke task manager (via three-finger salute), kill Explorer and restart it through New Task (and then restart the system). While it is in hourglass mode, it does seem that there is some network traffic going on (bottom left portion of network icon showing activity), but I'm not convinced that is every time.
Possibly related - sometimes, especially after returning to the machine after a long break (screen-saver - Aurora), some applications slow to the point that I application windows assembled - frames, outlines, panes, or an explorer window takes 10s of seconds to populate itself.
I use AVG & Spybot regularly, and have scanned using various online scanners and found nothing. I can't see anything obviously wrong in a HiJackThis scan.