Do you leave the external hard drive connected or just use it when you are backing up? Also did you back up the OS to it? If so, unplug it as computers don't like having the same OS on two drives at the same time. Otherwise write a small program that gives you the option of which drive to work from. I don't know what you used to do the back up with, but some programs will do a mirror backup which includes the OS. Others just back up the data files and such. You could possibly have registry problems now with all of the things going on. If you open the tower and find a bunch of small French Frogs in there, return the Computer to Dell as quickly as possible.
Ron
