In winXP, you can edit mp3 tags in the properties window of the file - Why doesnt it always let me do this? Sometimes i have far less tags available. The file is not in use or Read Only. This has baffled me for ages.
I use a program called Media Tagger to do this as it can read lots of files and insert these tags in them all at once based on the filenames.
I dunno about doing this manually in the Properties window although I have done this once or twice.
There are 2 or 3 different tagging standards in use. ID3v1 and ID3v2 being the most popular. I think ID3v1 holds less info than ID3v2 and I think ID3v2 can be set to read only.