I love colleagues like that, big help isn’t it?
Are you sure it’s definitely not a hardware / network problem? My first port of call would always be to check those first, try using perfmon or other tools to monitor the system performance and check for any bottlenecks that are unrelated to SQL Server – even a 3rd party application that could be hogging resource. If it’s definitely SQL Server, you really have a million and one things to go for if you haven’t a clue where it could be coming from… you might want to try using perfmon again on some of the SQL Server counters to check for any problems. Also, it might be useful to use profiler to record a good portion of database activity from various points in the day (particularly when it is slow) you can then see what code is being run and will be able to look at it to see if it looks particularly inefficient etc, this will also lead you down the route of checking your indexes etc to make sure they are well used and efficiently put together. Check when the statistics on the database were last updated (if at all) as this might also be worth doing, without them up to date you’re basically navigating across the country with a 1920s road map.
What version of SQL are you running?