Help with loading windows service pack 2

I just got 3 notices in my e-mail that I won those free music downloads from the Mt Dew Transformers promotion...but to get them I have to have service pack 2.
I've tried loading this several times before, but keep running into the same problem: it'll get through with the initial installing, but just about as soon as SP2 starts installing, a warning pops up saying, "access is denied" & then the whole thing starts uninstalling from there. I have checked my computer several times, I have all the minimum requirements, have run virus scans & adware scans, even a Dell support program that was supposed to have my comp running at max efficency.....basically just about anything I could think of, yet this is still happening.
Does anybody know if this is a common problem, or has had it happen before or something that might be able to help me?
I've tried loading this several times before, but keep running into the same problem: it'll get through with the initial installing, but just about as soon as SP2 starts installing, a warning pops up saying, "access is denied" & then the whole thing starts uninstalling from there. I have checked my computer several times, I have all the minimum requirements, have run virus scans & adware scans, even a Dell support program that was supposed to have my comp running at max efficency.....basically just about anything I could think of, yet this is still happening.
Does anybody know if this is a common problem, or has had it happen before or something that might be able to help me?