There are many situations that can cause a Windows PC to misbehave, crash, slow down, and generally not be as efficient as you might hope. One such situation is a shortage of working memory, or RAM. This is different to hard disk space, of course. Even if you have lots of spare disk space to store the things that you're not currently working on, you might still be running short of RAM because a certain program, utility, driver, document file or somesuch has been loaded.
If you want to troubleshoot RAM-related issues, or you simply want to know what's taking up all your memory, then RAM Map from the SysInternals team (which is now part of Microsoft) is a great place to start. It's a tiny utlity of less than 0.5 MB, is virus-free, runs on all recent versions of Windows including 8, and is free. You can get it from http://technet.microsoft.com/en-us/sysinternals/ff700229.aspx and it needs no installation - just unzip the .EXE file and run it.
As you can see from the screen shot below, my antivirus program (Sophos, and specifically savservice.exe) is currently using 153 MB of RAM where as Dropbox is using 48. It all adds up.