I am posting this so others who may experience the same issues could find a quick solution.
I spent over 6 hours today trying to install the RealPort driver and PortServer TS 4 on windows 7 x64. For some reason, it kept crashing my machine with the following message:
IRQL_NOT_EQUAL_OR_LESS
After poking into the dump files and searching on google, it turned out that the actual file that was causing the crash is SYMTDIV.SYS
A quick search showed that this file is part of Norton AV.
Uninstalling NAV solved the problem and I was able to install RealPort. Then installed NAV after that and all is good.
I hope this helps.