Thanks to everyone for their replies. As usual, I've learned a lot!
I followed Richard's suggestion and decided to share another directory on machine A. Lo and behold, I could access this on B and C, and could also access the other 2 directories that I was unable to access before. I don't think that I did anything different to before, so I'm stumped as to what's happened. It could be an issue with the wireless doobries, as trying to access the internet in machine B was slow to the point of impossible tonight, but machine C which sits beside it was at normal speed.
Just for the record, I'm running a workgroup, no domain. I've set up the sharing through the folder properties tab, ticking the 'share this folder on the network' box.
Being fairly paranoid about security, I hope that my set up is ok, Mac the Knife. I've got the broadband cable modem going in to a Linksys router, which then has the cable to machine A, with Linksys USB doobries on machines B and C. I don't use the XP firewall on any machine, but do have zone alarm. Whilst probably the most sensitive information anyone could get from my PCs are my numbers for next week's lottery, I'd rather that nobody else sees them because they'd only laugh when I fail to win a tenner yet again. I do use the 128 bit encryption over the wireless.
In a few months, so long as I can still share directories, once I'm comfortable that it hasn't suddenly gone back to a lack of access, I might try mapping the drives, as it looks more secure.
Many thanks to all once again.
cur