All other things being equal, windows 7 is more secure for (at least) one important improvement: the ability to perform most, if not all, of your normal activities without being an "administrator" user. While XP does have the notion of a non-administrative user (limited user), it is half baked, and in a non-corporate (non-domain) environment, ones ability to actually do work on the computer without being logged in as an administrator is sorely lacking.
Windows 7 has improved this, and can operate in a manner similar to OSX in that you can operate as a non-privileged, non-administrator user all the time, and temporarily upgrade your permissions to accomplish a specific task, leaving your default, non-administrative access in place otherwise. Win7 is slightly less intuitive here than OSX, but if you're running both on the same machine, I am sure they will make equal sense (BTW, take a look at VMWare Fusion as well as Parallels - both do the same things, but each has its own strengths and weaknesses. Depending on what you're going to be using your Windows virtual machine for, make sure the virtualization product you choose is well capable of that)
I use macs and osx almost exclusively these days, and develop web application software for windows using Win7 in a VMware virtual machine - it works, and works fairly well. There is virtually (no pun intended) no reason to install a new copy of windows XP in 2011. The ultimate edition even has a way to run virtualized XP under windows 7. I would not recommend using that when you're already virtualizing Win7. But, if you are concerned about some older software that may require XP, bear in mind that you can create multiple virtual machines using either parallels or vmware, and could set up a small XP virtual machine as a bit of a security blanket, albeit one that may open you up to some security vulnerabilities if not properly maintained.
Thank you for your detailed answer.
I haven't heard of
VMWare Fusion and will check it out.
Most of my concerns about Windows XP vs Windows 7 relates to my inexperience with Windows 7 and security.
I understand your logic about
"There is virtually (no pun intended) no reason to install a new copy of windows XP in 2011." And I'm beginning to understand the need to run most of my time in "non-administrator user" mode, for combating MBR Rootkits.
I guess at my age (over 50) and my time demands, it's hard to let go of a familiar system, especially after hearing so many of my friends shoot down Windows 7, due to their headaches with some of their "needed" programs.
Thanks again for your time.