John, before blowing away the OS and relying on a single backup from OneCare, I would recommend that you check to make sure that you are not running any conflicting programs with OneCare. Did performance go bad after you installed OneCare? If so, it likely means that something is conflicting with it.
On the other hand, if you are up to it, go ahead and start fresh. I would *not* rely on a single backup of your critical data, though. I would recommend that you copy your critical data to an external disk, another PC over the network, or burn the files manually to CD/DVD. The OneCare backup is reliable, but in the event of a problem, I would want to have an alternative for restore.
If you wish to have some help from support to review your system performance and insure that you are not running conflicting software, contact them -
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve