According to Alexander Weber from Map Solute:

I found a solution how to detect if SP2 for IE 6 is installed and though maybe that is usefull to you too. It can be detected within the user-agent, refer:

http://msdn2.microsoft.com/en-us/library/ms537503.aspx#UARegistry

''For example, an "SV1" token appears in the user-agent string of a Microsoft Windows XP user who has installed Windows XP Service Pack 2 (SP2).''

Therefore you might check the user-agent for the tag "SV1" to detect service pack 2 beeing installed.

We had the same problem to solve and one important customer had IE 6.0 SP1 installed, but we didn't want to disable compression at all for SP2 users, which are the majority I guess (because compression works fine for those users).