XP Pro serious lag problem

I have XP pro, service pack 2 which has been running fine for months.
Recently noticed an error on startup relating to a dll for my device driver,
so I re-installed software.
Since then have had problems. One of the dual core processors is constantly
running at around 40-60%. Further investigation reveals that DPC's (Deferred
procedure calls) are using up this processor time. This is causing serious
lag to the system and hard drive activity is greatly increased. I am truely
puzzled. I have even tried re-installing the OS, which seemed to work for a
couple of hours, then back to the lag. I am wondering if I am may be having a
hardware failure which is causing the problem, possibly my boot drive?
although system seems to be working fine in safe mode.
Can anyone shed any light on this problem? Or if not, can anyone tell me how
I can find out which devices are usings the DPC's?
Thanks