Sometimes, Mcshield.exe hogs more than 100 MB, but that's unusual.
Does it happen in particular situations, like booting up the computer, or during/after updates, when you acces certain webpages ? Do you have any other (security) software that might conflict with McAfee ?
McAfee just tends to consume a lot of resources (CPU and RAM) sometimes.
3. Launch Windows Explorer and delete all McAfee files in "Program Files" but especially in "C:\Document and Settings\<user>\Application Data" and "C:\Document and Settings\All Users\Application Data". You have to set Windows Explorer to show all hidden files and folders (Tools > Folder Options > View tab). Just delete the McAfee folders even if they are empty in all the "Application Data" folders. Can be more than one! In Vista: C:\Program Files, C:\ProgramData and C:\Users\yourname\AppData\Roaming
4. Reboot and redownload/reinstall your McAfee products directly from your account at the McAfee web site.