...I invested in RM6 only to find that, as a user of XP with its "old" IE version, I cannot access Family Search from within the program although the blurb for RM6 says that it is compatible. with XP (which itself is now under threat of cyber attack)...
Like you I also must be an XP hardliner. I use XP Professional (SP3) that has been updated through April of this year - note that MS is continuing selected monthly updates for a limited time. Nonetheless, I am quite comfortable using XP with Firefox as my browser (it works with FamilySearch).
Although I have a Win 7 laptop with Office 2007, I prefer XP and Office 2003. I find them much easier to configure - perhaps due to my familiarity with the older programs. Possible one day a program I need will not work with XP, but until that happens XP is my OS of choice.
For secuity and peace of mind reasons, I also clone my entire hard drive weekly as a backup measure should something get by my firewall, AV, and Malware programs. Doing that has saved me endless restoration hours - it only needs to happen once and it pays for itself to be back in operation in 5 min.