The best diagnostics I've seen are those that boot their own OS from
a floppy. However, they are not "free" either as beer or speech. One
I have used in the past is PC-Technician. Do a Google search for
"hardware diagnostics" and browse some of the 20,000+ hits a bit
then come asking for recommendations.
For PC-Technician see: http://www.yqcomputer.com/
If there is a "free speech|beer" release of a diagnostics program
that comes close to PC-Technician I've never found it.
Gene (e-mail: gene \a\t eracc \d\o\t com)
Linux era4.eracc.UUCP 2.4.22-28mdkenterprise i686
08:27:05 up 143 days, 2:25, 12 users, load average: 0.36, 0.11, 0.03
ERA Computer Consulting - http://www.yqcomputer.com/
eCS, OS/2, Mandrake GNU/Linux, OpenServer & UnixWare resellers