Internet Use Dramatically slows down PC

In the last five days I have noticed that whenever I use the internet to do
anything more demanding than browse websites - such as watch Youtube videos
or download files - that my entire PC turns into a stuttering wreck.

This isn't a subtle change, but a huge shift in the way my PC handles
typical workloads and it is incredibly frustrating. It is also not a simple
case of my ISP letting me down - other household PCs are not suffering in the
same way,

When playing games or editing video I can see no difference in performance -
this appears to be a solely internet related issue.

It is also not an internet explorer issue, as Google Chrome and Firefox
cause the same problem.

I have performed dozens of seperate virus/malware/etc checks and I have
found nothing untoward. There are no processes running in taskmanager that
seem to either hog too much memory or that should not be running.

I have restored my PC to an earlier date and this also made no difference. I
have also checked my equipment and I can confirm that the cables, network
card and so on appear to be in good order.

All my drivers, windows updates and so on also appear to be in good order.

My only thoughts on the matter are that either:
a) I have a virus that I have not been able to detect with any software thus
far
b) A file has been corrupted that can not be corrected with system restore

For the record, my system is as follows:

Windows XP
Intel E700 2.7Ghz
2Gb Ram
250Gb HDD
256mb ATI 4850
Ethernet connected internet

However, I STRONGLY suspect that this is not a hardware error as my PC has
only recently been fully checked over and - for example - when playing a game
online (something that's intensive in every sense) there is no performance
issue.

Also, I should add - because someone will inevitably say this! - I am well
aware that streaming online video will use SOME of my system resources, but
this has only just started to happen and it is using up ALL my system
resources.

Does anyone have any suggestions that might help?

Thank you for your time,

Adam