The most important factor is not the CPU, as ANY core 2 duo out there is much more than enough to run windows. Memory is the most important factor, but with 2 Gb it should be more than enough. I have an iMac with 2 Gb and I use windows xp under VmWare fusion. I partition my memory with 512 mb for win xP and 1.5 Gb for OS X, without any problems whatsoever. Win xP runs just fine with 512 mb of ram allocated, if you want more then just split the memory with 1 Gb of RAM allocated to each OS.
I wouldnt recommend Vista as it is resource hog that will consume most of your ram just for basic system resources without much direct benefit to you. So use windows xP instead.
With this in mind the macbook air should be more then capable of handling windows under parallels or vmware.