I'm curious, was the manual update succesfull ? On my machine, current DAT 5469.
Thinking this through shouldn't hurt.
But the way I see it: you can't go on like this.
If uninstalling and reinstalling McAfee does not work, then, for as far as I can understand, your system has been corrupted, or you are infected.
In which case, aside from asking for help with removing malware manually, like posting a Hijackthis log at, for example, Castlecops, the only solution would appear to be reformatting your harddrive and reinstalling everything. If you don't know how to do that, make sure you have someone who can assist you with this, it can be tricky sometimes.
Just as an afterthought: do you have an account at McAfee directly, or does it go via Dell or some other company ? That may make a difference.