At first I would say that something has gone wrong and the process is infact eating up 100% of one core. Check the task manager. If that's so I would guess something is corrupt with McShield.exe especially if the error returns.
Try to kill the process and restart it.
The two things that I found are: setting under Windows System Performance the setting from Programs to Cache eliminates the WINLOGIN.EXE issue - it's now cached so when it's called, no disk read is done. I then found that running MC Virtual Technician fixes the MCSHIELD performance problem on my XP system but it reappears the next time an update is done (I have auto-update disabled). So in my case, it looks like the updater is the cause to whatever the MCSHIELD performance problem is.
Which exact version and patch level do you have installed? You can see that if you rightclick the mcafee icon in the notification area and select the last option.
I looked and the last option is Help. If you are asking for revisions: Sec Center 11.6 and Virus Scan 15.6. It's a brand new installation. The stores (Staples and Office Max) didn't have the 3 user Anti-Virus Plus 2013 licenses boxed, so I bought a 3 user license as a download from Office Max.
Update: Yesterday, changing the Windows XP setting from Programs to System Cache eliminated the WINLOGIN.EXE CPU bandwidth problem, but the MCSHIELD CPU bandwidth problem was still there. MC Virtual Tech would fix the MCSHIELD performance problem, but the updater would just put the problem back after an update. However, after a manual update this morning, the MCSHIELD CPU performance problem was also gone and I am running 98% idle. I checked the Consumer AV product rev numbers and they are still at 11.6 for Security Center and 15.6 for Virus Scan. Hopefully the good performance will last past the next update. :^) My VISTA machine running dual cores didn't see any of this, so this is a single core/thread issue. Dual cores don't seem to see it. Time will tell. :^)