Slow start after up/down grade

Hi,
I have a client with P4 3GHz, 1 GB dual channel memory,40 & 120 GB HDD, partitioned 2 and 4 respectively, WinXP pro. Client not computer savvy, but uses it very effectively as a tool.
Problem: He upgraded to SP2 – computer spat the dummy. Friend got him back to previous version, (somehow). Various problems, which I have solved, except for boot, which is now very slow. Checked programs that start on boot and nothing untoward there.
Ran AdAware, Registry Mechanic, no joy. Checked system processes 99% idle.

Have checked out TWEAKXP website and too many options to try off site.

Any ideas would be welcome.