From what I read you won't be able to use iCloud on Windows XP, have to use Windows Vista or 7 in order to install iCloud.
iPhone 3 GS should work with iCloud but again will not be able to get full benefits from it until you upgrade to Win 7 on your computer or have a Mac.
You can actually get iCloud control panel to work on Windows XP with a little hack, that'll give you synced access to Contacts, Calenders, Reminders and Bookmarks. It also can be used to set up your me.com mail. Photostream doesn't work though.
The iCloud setup hack for Windows XP:
- Download iCloud Control Panel for Windows.
- Open iCloudSetup.exe file with WinRar or 7-Zip and extract.
- Navigate to and open the iCloud.msi or iCloud64.msi (for 64bit systems) with Orca.
- In the left table select LaunchCondition. Then change in the right table “VersionNT> = 600” to “VersionNT> = 200” and Save.
- Run modified iCloud.msi and install.
- Run iCloud Control Panel, located in the Windows Control Panel, and set up as you want.