Expanding on Jim's reply, you can't disable OneCare at startup - it isn't designed to work that way. It provides protection all the time. As Jim noted, what you describe as your experience for slow loading of OneCare, impacting the PC's performance so significantly, indicates that other security software is still installed and active on your PC.
I once worked on a computer that was booting slowly and discovered no less than 3 full antivirus programs installed and trying to run, though only one of them appeared to be active in the System Tray once XP had booted up after 30 minutes.
I recommend that you contact support to have your situation investigated and resolved.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve