• Reboot machine and attempt to rerun backup.
• Split the  job if multiple drives exist. Creating a separate backup job for each partition.  Note: This is not recommended when backing up Microsoft Exchange or SQL servers if the databases and logs stored on different drives.
• Change the scheduled time of backup so no conflicting application is running during that time.  If this fails, perform a clean boot and rerun the backup.  More information on performing clean boot:    http://support.microsoft.com/kb/310353  
• Increase physical memory.
• Increase Windows swap file size and reboot.

If the above Steps did not Fix the Issue then Run Disk De-Frag (Run De-Frag more than twice Just in case if the Disk is heavily Fragmented) on all the Drive Present and Run Check Disk with switch /f and /r.