Should I install antivirus in Windows 7 RC XP Mode?
Solution 1:
Take a look at the Microsoft Q&A on Windows 7 and XP Mode
Specifically they say:
... It is not pre-configured with anti-virus or anti-malware software, which is recommended.
Solution 2:
Yes you should, you don't want your VMs getting infected with nasties.
VMs are just as vulnerable to viruses and spyware ans real computers. Odds are you are going to run some old random programs from the internet on your XP VM. You want to catch the infection as early as possible. If your AV on your Vista machine misses you want to catch it on the XP VM.