No idea what causes it, but a reboot and installing mozilla firefox to use instead of explorer should fix it.
The reboot will. FF wont.
? I'm genuinely interested in the above.
In my experience - six computers in the office, all same-ish spec - the ones which run IE need rebooting to fix this problem about once a month/six weeks. The FF ones, about once every six months
Well, something wrong with your IT manager then, as they should get monthly patch updates, which invariably require a reboot. Also, I'd like to think that any IT manager knows that FF itself needs updating very regularly, due to suffering all the same security flaws that plagued IE years ago.
IE is pretty rock solid - as is FF, apart from its lack of maturity - and not known to be a resource hog. However, like other browsers, plugins are not under the control of the browser vendor, but do run under the same process. 99.9% of the time, when a browser process starts to hog resources, its due to a plugin.
If you have an IE process misbehaving, check the plugins, or simply uninstall them. If your IT manager is stupid enough to allow the users to run with Admin rights, there will likely be all manner of crap firing up. Nobody, the IT manager included, should be running with Admin rights. In addition, check for any shell extensions, as many of these frequently make use of IE.