Often the reason for verification failure maybe a failing hard drive, or running other programs, such as your anti virus software, at the same time as the SFC.
Try SFC from a clean environment by stopping third party programs and services from running.
310560 - How to Troubleshoot By Using the Msconfig Utility in Windows XP
http://support.microsoft.com/kb/310560
How to troubleshoot a problem by performing a clean boot in Windows Vista