Is this Windows 7 SP1 and fully updated? Also what security software was there previously and did you uninstall and run any applicable removal tool?
If McAfee was installed try: http://service.mcafee.com/FAQDocument.aspx?lc=1033&id=TS101331
Then reinstall from your account
Also if you have Malwarebytes installed, temporarily uninstall it as it causes the installer to fail.
I'm having the same problem on two computers, Windows 7 Home and Windows 7 Enterprise. I have two 3-user subscription, the last time I installed it was around June 30 and things were perfect. I tired everything possible, the abovementioned instructions, modifying folder security credentials, installing to a different than the default folder, everything. It's just frustrating.