Spyware/Adware/programs running in the background
are all potential problems.
Look at your processes under the Task Manager, google any suspicious ones to see if they are legit or spyware/adware/virus.
Also memory intensive programs like games or in somecases photoshop may limit your abililty to open several windows/programs due to lack of ram.
If you have a lot of startup items they may be contributing to the problem.