|NOTE that sometimes the resident module(s) of anti-malware programs - a part that runs all the time scanning for suspicious activity - will interfere with the proper installation of software, the software will not install properly, and you may get no indication of that at all while installing the software.|
To avoid that possibilty, you should always DISABLE the resident module(s) of anti-malware programs, BEFORE you install third party software (software other than most Microsoft Updates, etc., that did not come with Windows ), especially when it's a major or complicated software package.
E.g. if you are using the free or paid version of AVG, you should disable the Resident Shield in AVG's 's settings in Windows (in AVG 2012 that's done under the title AntiVirus). In Norton (Symantec) products, there may be several things you need to disable, or set so they don't load for a specific short amount of time.
If you don't know how to do that, tell us which anti-malware software you are using.
When you are sure the software has installed correctly, re-enable the resident module(s).
In your case, either disable NPAV's resident module(s) or un-install NPAV BEFORE you install IE 8.
Note that if it's a Norton / Symantec product, you probably MUST un-install it.
It's also a good idea to un-install IE 7 before you install IE 8.
Hoiwever, you can't un-install IE 7 (or 8) if it was installed before you installed the Windows SP3 updates.
If that doesn't help, tell us what exact error messages you're getting when you try to install IE 8.
Note that if you had any of these problems only AFTER installing Windows SP3 updates, you must un-install the SP3 updates, then disable any anti-malware programs resident modules or un-install them BEFORE installing the SP3 updates again, If you do that, it's recommended you un-install IE 7 or 8 BEFORE you install the SP3 updates, install it after installing the SP3 updates, otherwise you won't be able to un-install IE 7 or 8 if you have problems with them later.