In Windows XP, F8 should always work for booting to Windows Safe Mode. It's built into WinXP. (KB 315222)

Some OEM builders may add another key that can be used, and some BIOS setups may allow something different. In theory, an OEM could change the F8 to something else, but there'd be no good reason that I can think of for doing it. Earlier Windows versions are much more subject to variations, with Esc, F2, F6, and F8 (and probably others) having been used.

In WinXP, however, it is NOT GENERALLY TRUE that you'll see a startup or post screen that will tell you what key to use, since WinXP generally skips the POST screen and also mutes the Startup Beep Codes. You can generally choose (in BIOS Setup) to have this preliminary stuff shown, but the default WinXP boot settings often omit it.

Another "peculiarity" to watch out for, is that while some BIOS chips support a USB port, some - apparently quite a few - don't. If your BIOS doesn't support USB, a USB/wireless mouse may not work in Safe Mode. You may also be unable to access USB devices, like external hard drives in the basic Safe Mode(s).

There apparently are a very few BIOS chips that don't support a Serial port, and with these even the common "wired" mouse may not function in Safe mode, although I don't believe this is common.

John