|Are you sure you disabled the right thing?|
Scroll down - it shows where you can enable / disable Real time protection:
Are you SURE you disabled the resident module(s) for ALL anti-malware software ?
If you ARE sure, there are some third party software firewalls that have a resident module that causes the same installation problem - do you have any such program installed ?
Spybot free and paid, AdAware free and paid, Avast! free and paid all have at least one resident module.
Windows Defender has a resident module - installing Microsoft Security Essentials automatically disables that when it's installed if Windows Defender is present, at least it does in Vista and Windows 7.
Windows Defender is built into Vista and Windows 7 and it can't be un-installed, but it can be disabled from running in Windows Defender's own settings.
Malwarebytes free has no resident module (it does but not one that causes this problem) but it's paid version does and that may cause this problem.
If that doesn't help I have no idea what the problem is.You could try searching on RealPlayer's web site, or searching on the web, for possible solutions.