I've been using my Smartphone (T-Mobile MDA Vario II / HTC Hermes / HTC TyTN) with Windows Mobile 6 (WM6) for some time now and had few issues with it until recently.

Being without internet whilst moving flats, I've resorted to Mobile Internet Explorer for most of my surfing. After digging around (but not before wiing out half my messages in an attempt to recoup some storage) it appears that IE doesn't have a limit to the amount of temporary files it store. As the phone only has 64MB onboard storage, space is at a premium.

To clean up the debris it leaves behind, open up IE, head to Menu -> Tools -> Options. Under the Memory tab, hit the "Delete Files" button. In my case, it removed 15.5MB This has been tested on Windows Mobile 6 and I'm assured it's the same process for Windows Mobile 5.