Computer suddenly hanging/crashing

My computer (running XP Home / SP2) has begun hanging, bunking out of
programs and even crashing. Anti-virus is Avast, which I've been using
for years and am totally happy with - never had any infections.

I installed 2Gb of memory a week or so ago, giving a total of 2.5Gb -
computer is configured for up to 3.5Gb. Things went fine till last
night, while watching 4oD.

The only other thing I've changed is to uninstall the old 4oD player,
which is no longer needed, and prevent the (I was told) Kontiki helper
program Khost.exe from loading at startup.

Examples:

*Hanging*: capricious and application-unrelated - even just surfing
with Mozilla or I.E.
*Bunking out*: Second Life
*Crashing*: Channel 4's iPlayer (4oD)

It's getting really boring and means I can't do anything much. It even
hung when I was setting up this HTFC account. Any help/suggestions
appreciated; thanks.