Phil,
Open IE7 -> Tools -> Internet Options.
In the General Tab look for Browsing History. Click Settings and in the new window have a look at how much Disk Space is allocated. What ever it is, increase it, to start with to 250, You have plenty of memory so it won't be a problem.
Click OK and then go to:
The Advanced Tab and in the Settings window scroll down to Security and ensure there is a tick in "Empty Temporary internet Files when browser is closed". It sounds as though that will already be ticked as the file appears to be empty when you first start up, but check.
Try that for today and see if there is enough space.
If it re-occurs then repeat the first part and increase to the space to 500.
dave