Quite some time ago, I ran into a problem where SQL Server (back then I think it was version 7) would consume all of available RAM forcing other applications to the bare minimum.  Since that time I usually always tell SQL Server exactly how much memory it may use, otherwise it can bring you system to a crawl.

To set, bring up the Properties of the SQL Sever you have open (in Enterprise manager, SQL Server Management Studio, etc) and select Memory.  By default it consumes all available memory.  Set this to a limit where your system is still fully functional but not to choke the server.  On my development machine I have this set to 256 MB.  

Also, you need to keep in mind that if you have multiple instances of SQL Server, you need to configure the RAM on each of them.  If multiple instances are running at the same time, make sure they will not run out of memory, as the amount you set on an instance is the amount that one instance can use and not combined.