The 531s Micro-ATX Desktop comes with Athlon 64 X2 3800-6400, later processors not supported.
Have you updated bios/clear cmos lately, could be a corrupt bios which has changed FSB down to 100 instead of 200.
Also if cool-n-quiet is enabled, you must have the driver installed for it to auto-change FSB to full when it is needed.
Use cpu-z to verify settings within windows.
Info @ http://support.dell.com/support/edocs/systems/inspd531/en/om/appendix.htm