Regarding whats running on my comp

Recently my comp has been getting slower and slower is their any way i can stop certain things running and if i can how do i know what i need to keep and what i can stop or remove.
Do you know what im trying to say not the programmes as such but whats running in the background how do i find out whats running and then how do i know what to remove.

Hope you can understand what im trying to say, i hope so anyway.
If not dont worry just thought id ask as my comps been getting slower and slower. Its slow anyway but it seems to take longer to come on and seems to be slower than normal.