This solution from 2008 had only been tested on Windows Vista. The number of negative thumbs on this entry suggest it will not work on Windows XP or Windows 7+.
Today for some particular reason my version of Microsoft Vista had forgotten what to do with ‘.msu’ files. MSU is a file extension used by Microsoft to package operating system updates that are not tied in with Windows Update.
Fortunately there is a quick and simple solution, but for many people this is not too obvious. We simply need to re-associate the ‘.msu’ extension with the Windows Update Standalone Installer. Thats the application included in Windows and is used to install these packages
Firstly you need to click ‘Select a program from a list of installed programs’ and press OK. The following prompt will then appear.
From here, just select ‘Browse’ and then press OK. Also make sure the ‘Always use the selected program to open this kind of file’ is ticked.
Now browse into your ‘Windows\System32\’ directory and look for the file ‘wusa.exe’, select it and click OPEN.
Now the Windows Update Standalone Installer will be an option within your Open With dialog. Make sure it is selected and press okay.
That’s it, now your .msu packages should be working again as normal.
The Developer's Tidbits 
sorry, i meant bobos fleese…
Hi Guys,
i am new to this site. I tried following your solution only to find that the WUSA.EXE file is not in the Windows/system32 directory (or anywhere on my pc) how do I get this back. I am Running Vista Basic.
Thanks
Steve N
Thanks Steve, helped me to get the file unpacked in XP.
use the expand command but try the /? switch to get info on how to use the command. I created a temp folder on the C:/ root with the .msu file I needed and it worked:
expand -r IE8-Windows6.0-LanguagePack-x86-ESN.msu -f:* c:/temp
Good old Microsoft strikes again. They make IE8 langauge packs available but apparently you can only install them in Windows Vista. By the way, I tried WoofWoof’s suggestion but I just get an error message saying there are no files to expand.
in xp,you can use
expand -f:msufilename destinationfolder
to extract the msu files. same in w2k3.
Windows Update Standalone or MSU files were only introduced into Vista and Windows Server 2008. They are not supported by XP; so no XP updates would be distributed or be used by this application.
http://support.microsoft.com/kb/934307
This works great for Vista, but what about XP?
I was looking for this solution for months thanks
No problems, I am glad I could be of some help.
Hey thanks a lot buddy 😛
much appreciated!!
most of the times the simplest of things appear too difficult…
am a beginner so need all the guidance i can at this stuff 😀
cheers!! n thanks again