I'm not a MaC or iCloud user, but I found your question intriguing, I found a link by Google to a blogger who claims to have successfully installed and using iCloud successfully as August 2012.
It seems the iCloud MSI installer performs an OS check and halts if it discovers XP.
The author used a well known trick to hotwire the MSI into skipping the OS check thus allowing ti install on XP.
She went further by even hosting those modified MSI for download in addition to her well written instructions in case your just in a hurry to start the music.
it is worth reading and may be the solution you need instead of the variables of upgrading an entire OS with anohter bigger version of an already inefficient OS just to get access to one application.
http://clarinerd.com/blog/software/icloud-for-xp/
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