This is not supported by Microsoft, and never will be. There may be issues with running the client on an unsupported OS. Use at your own risk!
The Lync system requirements list Windows XP SP3 as the minimum that is supported. This unfortunately leaves out Windows XP 64-bit as there is no Service Pack 3 for that version.
If you run the Lync Install on 64-bit XP you will get a nice error informing you of this fact.
Officially Windows XP 64-bit is not supported by Microsoft, but if you don’t mind running in an unsupported scenario, then you can use the following steps to work around this. You will need at least one system that is a supported OS for this to work.
- Run the LyncSetup.exe on a supported (preferably 64-bit) workstation and it will indicate it is unpacking some files and then prompt you. Leave the window alone and navigate into your Program Files (x86) folder and there should be a new folder called OCSetup.
- Copy the entire OCSetup folder and move it to the Windows XP 64-bit workstation. You can cancel the install on the first workstation now.
- On the Windows XP 64-bit workstation, install silverlight.exe, vcredist_x64.exe and vcredist_x86.exe that are included in the OCSetup folder.
- Install MSXML SP1 if you do not already have it installed. Download link.
- Run the OCSetupLauncher.exe to install the client.
I was able to log into the client and verify IM, presence, and conferencing appeared to work fine.
It does appear that the standard (self-healing) launch links have an issue in XP 64-bit. Each time you launch Lync through the shortcut it will run a repair install. You can work around the issue by replacing the self healing communicator short cuts with standard shortcuts to communicator.exe.
There may be other issues with running the client on this OS, so your best option will likely be to just go ahead and upgrade to Vista/Windows 7.