Question: Where can I download all the Windows XP Service Packs?

Description: Microsoft does not have a easily located singular location to download SP1, SP2, and SP3.

Answer:
You can check out the Windows Service Pack Center:
http://windows.microsoft.com/en-US/windows/downloads/service-packs
Which has all updates listed for Windows XP, Vista, and 7.

Or, you can download from the individual Microsoft Download Center locations:

SP1: (Sp1a Express) http://www.microsoft.com/en-us/download/details.aspx?id=19751

SP2: http://www.microsoft.com/en-us/download/details.aspx?id=28

SP3: http://www.microsoft.com/en-us/download/details.aspx?id=24

Keep in mind, download Service Packs manually will be much larger than installing them via. Windows Update

Need the 64 bit version?
SP1 x64: There is no SP1 for XP 64 bit. This is because XP 64 was based on the Windows Server 2003 SP1 codebase (essentially, it already has SP1 in it).
SP2 x64: http://www.microsoft.com/en-us/download/details.aspx?id=17791
SP3 x64: http://www.microsoft.com/en-us/download/details.aspx?id=17791

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