My SQL Server CPU has been at around 90% for the most part of today.
I am not in a position to be able to restart it due to it being in constant use.
Is it possible to find out what within SQL is causing such a CPU overload?
I have run SQL Profiler but so much is going on it's difficult to tell if anything in particular is causing it.
I have run sp_who2 but am not sure what everything means exactly and if it is possible to identify possible problems in here.
To pre-empt any "it's probably just being used a lot" responses, this has only kicked in today from perfectly normal activitly levels.
I'm after any way of finding what is causing CPU grief within SQL.
I assume due diligence here that you confirmed the CPU is actually consumed by SQL process (perfmon Process category counters would confirm this). Normally for such cases you take a sample of the relevant performance counters and you compare them with a baseline that you established in normal load operating conditions. Once you resolve this problem I recommend you do establish such a baseline for future comparisons.
You can find exactly where is SQL spending every single CPU cycle. But knowing where to look takes a lot of know how and experience. Is is SQL 2005/2008 or 2000 ? Fortunately for 2005 and newer there are a couple of off the shelf solutions. You already got a couple good pointer here with John Samson's answer. I'd like to add a recommendation to download and install the SQL Server Performance Dashboard Reports. Some of those reports include top queries by time or by I/O, most used data files and so on and you can quickly get a feel where the problem is. The output is both numerical and graphical so it is more usefull for a beginner.
I would also recommend using Adam's Who is Active script, although that is a bit more advanced.
And last but not least I recommend you download and read the MS SQL Customer Advisory Team white paper on performance analysis: SQL 2005 Waits and Queues.
My recommendation is also to look at I/O. If you added a load to the server that trashes the buffer pool (ie. it needs so much data that it evicts the cached data pages from memory) the result would be a significant increase in CPU (sounds surprising, but is true). The culprit is usually a new query that scans a big table end-to-end.