OneCare should run fine on a 4 year old laptop, as long as you aren't running on the absolute minimum system requirements. I would anticipate that a 4 year old laptop is running XP and has at least 512 megs of RAM. If you have less memory than this, it would be slow.
Of course, the slowness may have nothing to do with OneCare and could indicate a hardware problem, but the most likely cause is that the McAfee software has not been fully removed and part may still be running.
McAfee Removal Tool - http://www.majorgeeks.com/McAfee_Consumer_Product_Removal_Tool_d5420.html
If that doesn't resolve it, please contact support for some help.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve