There are a number of possible reasons for drive space being consumed. it could be due to expanding temporary files located here:
C:\Users\[windows login ID]\AppData\Local\Temp - to see this folder, you need to have Hidden Files visible via the Folder Options.
Up to 15% of the drive can be used by System Restore for Restore points.
Since you are using Home Premium, you aren't losing space to Volume Shadow Services, which is a feature of Business and Ultimate editions of Vista, where changed files are retained as "shadows" allowing you to recover older copies of files.
Other than these, space is likely being consumed by a program or programs on the PC. Without knowing more about your system and what is running, it is impossible for me to direct you to any other specific location. I would suggest that you open Windows Explorer and navigate all folders, starting with your own User folder branch. Right click each top level folder and select properties to see what the size of that folder. You may be able to determine what is using up the space on disc in that manner.
I am sure, however, that this isn't a OneCare issue, though it would be helpful if OneCare was able to perform a more thorough "cleanup" of the disk to show you where your space is being used and what you could recover.
I'll add that the IE error is also not likely related to OneCare and I'd recommend visiting here for help with IE:
Internet Explorer 7 Solution Center
-steve
Microsoft MVP Windows Live / Windows Live OneCare Forum Moderator
