The pidgin installer (Source: http://developer.pidgin.im/viewmtn/revision/file/6eb9d70596f75e6d69aa71408632a5bc08aca65b/pidgin/win32/nsis/pidgin-installer.nsi\) produces an error when run on a 64bit Vista installation.
If I recompile it with 2.27, it apparently works.
The following bug report was filed by the user experiencing the issue: http://developer.pidgin.im/ticket/3689
I'm not sure what other information to provide, I don't have a Vista 64bit box to test on.
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Works fine for me on XP x64 and Vista x64. Winamp 5.5 also uses NSIS 2.31 and there are several forum threads about other issues, not related to the installer. So I assume they passed the installer stage.
I'd go with your guess of corrupted download.
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I asked the reporter to download 2.31 installer from another location and it worked fine, so this is clearly invalid. Sorry for wasting your time.