Well the first thing you could try, which seems to work in most cases.
Delete your profile and create a new one. Could be your profile got corrupted somehow.
You find it here:
My documents\My Games\Crysis\Profiles.
Secondly, since you had an issue with installing the first time and the first suggestion dont work, you could try reinstalling MWLL again.
Since I'm not much of a techie myself so I will thirdly suggest you try looking in some of the other similar posts on the forum here to see if there are something there that may help.
You never know, maybe someone had a similar problem and found a solution that you can use.

Oh and you better get an anti-virus program ASAP, or you just risk getting something on your computer that will trash everything.
And by not having one you may very well be a source of infection for others, even though it may not be intentional and you most likely dont know about it either.