I used MacKeeper malware software and now can't log in successfully. Help!

I used MacKeeper v2.12 on my Mac Pro to cure a strange "bug" but now can't log in successfully. Within MacKeeper, I scanned my hard drive; it came up with thousands of duplicate files and folders as well as tons of old or outdated applications. Sadly, I deleted them all.

Currently I'm logged on using the "Guest" ID (which allows me to access the internet... and you); guess all I need to do now is reset my password to allow me access to my personal ID.

How?

Thanks,

~ Laura