Interestingly I disabled wireless sync in iTunes for both my iPhone and iPad this morning and the problem has not occurred since (8am-2pm) and iTunes has been open the whole time and playing music for much of it. I have also synced my iPhone via USB and experienced no problems.
So from my somewhat limited troubleshooting, it looks like it could be potentially be a problem with wireless sync - appearing in the most recent iTunes update.
I do like using the wireless sync feature but if this solves the problem for the time being then I'll just have to leave that feature turned off, at least iTunes is usable with it disabled.
I hope this helps someone else and I hope that Apple are able to resolve this issue sooner rather than later.
A quick update.
My PC has been fine for a couple of days now since disabling wireless sync on my iDevices, so it does seem to have done the trick.
I'll be sure to provide an update if this changes.