My suggestion would be Windows 2008 or 2003 server running Terminal Services and Citrix XenApp. This would run you about $2000 in software costs for Terminal Services CALs and Citrix Licenses.
This runs on Windows, Macs, iPhones and iPads. I currently use it on all four platforms.
http://www.citrix.com/English/ps2/products/product.asp?contentID=186