If you’re using Oracle on Windows 2003 SP1 on a machine with lots of memory, you should install SP2 as soon as possible.

Check Metalink note: 464683.1 “Unexplained Database Slowdown Seen on Windows 2003 Service Pack 1”

and Microsoft Knowledge base articles:

• A program that uses structured exception handling may stop responding for several seconds in Microsoft Windows Server 2003 with SP1
• SQL Server 2000 or SQL Server 2005 may temporarily stop responding on a Windows Server 2003 Service Pack 1-based computer

I can barely remember how many times we noticed exact symptoms, without having a clue what is going on…part of the problem is certainly slow adoption of Windows 2003 in the first year or two after release, otherwise this sorts of bugs would crop out earlier. The MS decision to wait so long with SP1/SP2 release was also a bummer for MS customers.