Adding to Vocalpoints good response, to update all your PCs you can copy over all the media files, all the library files including custom fields, custom views -- almost everything so the MC experience on each PC is almost the same.
You might consider setting your own library location rather than using MC's default. This can make it easier to find all the library files and copy them to other PCs, and especially to make sure all the files are in an identical location (path/folder) on each PC.
A common technique is to assign the MC library location as drive M:. You can do this even if it is a path/folder, not a separate drive, in various ways such as Windows ancient SUBST command.
http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/subst.mspx?mfr=trueBut for max performance (especially if the library is large/growing) consider a separate physical drive called M:. This is how I do it. My desktop has a second internal drive devoted to my MC library. On my laptop, drive M: is an external USB drive.
Also, it's a lot easier and safer to manage a library by designating one PC as the MASTER. Do all your tagging and other work on this PC, then propagate the changed files to all the other PCs. I do this via a backup program (SyncBack SE) since it is smart about determining which files to copy. If you allow changes on multiple PCs, only some changes will survive, so that's not advisable.