The 2007 XPS 420 with Windows XP and Vista was never supported with SupportAssist? SupportAssist was not released until 2013. Restart, tap F12. Do you see Diagnostics there?
Hi, I'm having problems running the Dell SupportAssist diagnostics tool. It basically says it is taking too long. Does this indicate the computer is pretty much past it?  
XPS 420, having problems with running diagnostics