I ran into a hitch while trying to install the latest version of TortoiseSVN on my XP 64-bit Windows installation. Apparently, the entire world has forgotten about the existence of this variety of Windows. There's absolutely no mention of it anywhere on TortoiseSVN's installer page. The MSI installer failed stating that the version of Windows Installer that I was using was dated and that I needed to update my version of XP (x86) to SP3 or some other drivel like that. TortoiseSVN has no manual install option either. I did the whole Windows Update thing as well, to no avail.
So, I needed a solution. M$ in all their benevolence does provide a page on the Windows Installer redistributable which notes which files to download for which version. I suddenly recalled something (and don't quote me on this) about Windows 2003 x64 basically just being a server version of XP 64. I have a feeling that I've used this trick before. Off I went to download the version of Windows Installer 4.5 for Win2k3-x64. I downloaded it from this page which does some version detection. But you will have to carefully choose the Win2k3-64 bit version which should be named WindowsServer2003-KB942288-v4-x64.exe.
Once that was installed, TortoiseSVN ceased its complaining and installed fine. My laptop has lived to see another day. Hooray!