It could be any number of things.
The disks on this server could be slower.
This SQL 2005 server may have more customers on it than the SQL 2000 server did.
I’ve seen cases when this happens specifically on 32bit hardware because of the changes in the way that SQL SErver 2005 stores the execution plan cache is different from the way that SQL 2000 stores the execution plan cache.
You will want to contact your hosting company and see if they have another SQL 2005 server that they can migrate you to.