It isn't OneCare. OneCare will not resume a PC from standby to perform any function at this time.
Something else is causing the issue, which, as you noted, sounds like the PC is actually being rebooted, possibly due to an error. You may want to try the Microsoft Public newsgroups here: http://www.microsoft.com/communities/newsgroups/list/en-us/default.aspx
-steve
p.s. I'm moving your post to the general folder.