ahah!
Presuming that you kept the folder structure on the new pc the same as it was on the old one, and that the only thing that's changed is the "base path", you would select all files, right click, library tools, find/replace.
Click on the 'select none' button, then, in the list, find and select 'Filename'
In the find field, enter the old base path.
In the replace field, enter the new base path.
Click OK.
In but a few seconds, your library wil be re-linked with your music files.
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The steps above have been presented with a rather cavalier attitude that assumes you understand what you're doing and why
It assumes that:
all files that were on the old pc are on the new one, in the same basic folder structure.
that the library restore has just been performed and
every file in the library is, as far as MC is concerned, missing.
perhaps try one or two files to begin with if you are not brimming with confidence, and once you are happy, select the lot and do them all.
Note that, if you use this method to fix broken file links in the library, MC updates it's [filename] records, then checks the new location to see if the files exist there, which they do, so everyone is happy. If you change the [filename] field for a file that MC has located and linked to, MC will actually move that file to the new address you have given it.
-marko.