I haven't tried Parallels on a Mac, but the Apple Boot Camp Beta 1.1.2 works very well, and it is a free download from Apple. Also, Apple says it will support Boot Camp in Leopard, the next operating system.
There is no intervening software between the XP operating system and the cpu with Boot Camp. To switch back to an Apple OS, Boot Camp inserts the usual startup plugin in XP, and the XP operating system is shown as an option in startup preferences when running the Mac OS. You can drag a text file from the XP partition (32GB or less) to a Mac partition and open it with Text Edit. Wireless mouse and keyboard will work in XP. Ethernet and wireless connections to the internet work in XP the same as in a Mac OS. I have measured continuous transfers in XP from an external hard drive (Maxtor One Touch) to the USB port at 20MB/s. I have downloaded the picoscope demo and tested it in XP Pro, but I haven't tried an actual Picoscope with a USB port. I'm waiting for more information from Picotech before I buy.
If you try Boot Camp, be sure and download the intructions and print them out and read them before installing XP SP2 (service pack 2). If you are using a wireless mouse and keyboard, you should also read "Boot Camp Beta 1.0.2: Using the Apple Bluetooth Wireless Keyboard."