Performance with SQL2005 db migrated to SQL2008

We have just moved our 2005 databases to 2008.
One query is giving us a problem. Its taking a lot longer to execute.. and has gone from seconds to hours. Nothing has changed in the database. The wierd thing is that it runs fine on a different test system also with 2008 installed. The difference between the the test system is that it has 16 CPU's (4cores x 4CPU) whereas the production machine only has 8 CPU's (4 CPU's x 2 cores). The production machine is exactly the same machine as before, we just flattened it and put on Win2k8 and SQL2008; and the same query was working fine on it.
One interesting difference is that when displaying the execution plan, we see parallelism occuring on the pre-upgraded prod box, and on the test box, but not on the new prod box. Parallelism options are set to default on all machines.
Any clues as to where to look to track this down.

[quote user="GMason"]Any clues as to where to look to track this down.
[/quote]
Please chck this: http://www.microsoft.com/downloads/details.aspx?FamilyID=66d3e6f5-6902-4fdd-af75-9975aea5bea7
Thank you,
Sandy.

Are the two systems running the same edition of SQL and Win 2008? Do they have the same amount of RAM? Same/similar disk subsystems?
Are you running in SQL 2005 compatibility mode, or have you enabled 2008 mode?