WinXP as non-admin
Using WinXP as nothing else than administrator is a horrible experience; many programs simply refuse to work or throw errors all over the place.
Being logged in as limited-rights user *is* good for security though; for some reason the wireless configuration utility refuses to work; no internet as a result.
Though completely secure I opt for Admin + Dropmyrights instead. (http://msdn.microsoft.com/ security/securecode/columns/default.aspx? pull=/library/en-us/dncode/html/secure11152004.asp)