dxdiag = direct x diagnostic tool

msconfig = sort out startup problems

ipconfig/all = gives you details of you network card inc ip address

regedit + regedit32

CHKDSK = check disk

netstat -a = find all those spyware traces

IPCONFIG /all (Display full configuration information.)
 

IPCONFIG /release (Release the IP address for the specified adapter)

IPCONFIG /renew (Renew the IP address for the specified adapter)

SYSTEMINFO (WinXP only and my favourite one plenty of info here)

Format C/u: =Formats the COmputers Drive C beyond recovery

Net user guest password = This is a nice one. Microsoft will have you believe the guest account has no option for a password but there is