I use this machine as a PVR and media extender synching my music and pictures to it from the home "server" machine where I hold all media centrally.
I also had this problem with my offline files and the fact my system disk was showing 20BG free after scavenging all the dross files out of it. I was unable to account for or find some 60GB of files but nowhere could I find the culprit until I downloaded treesize, logged on as admin and then ran treesize as admin. Then I saw the 60GB CSC files which equated to my offline files.
So I followed the advice above re. offline file disable and reduce file limits but vista would not let me release the offline cache. So I tried to take ownership and delete them manually but even though I own all of this stuff, in a strange HAL-like Dark star way my machine would not allow me to delete my own files.
So I followed a link to Microsoft
You may be unable to delete the cache of offline files from the client-side caching (CSC) cache on an offline Windows Vista-based client computer where there is apparently a hot-fix for precisely this problem. I downloaded, unzipped and ran it but it gave me a message "The update does not apply to your system".
So here I am with a vista rotweiller which will not let me delete my own files. Not happy