If you are not sure of which processes to end then just google each one. An example of a process is "jusched.exe". A good site that will return easy answers to your search is liutilities.com. It usually comes up in the first few results in a google search.
When you end a process, it will start back up when you reboot. In the event that you end a process that needs to be running, just reboot. To stop a process from starting when you reboot, go to Start\Run and type in "msconfig" (without the quotes). Then go to the "Startup" tab and uncheck anything you don't want to startup. Again, google each one in order to know if it is a necessary process or not. After you reboot there will be a confirmation box that comes up on your desktop. Check the box and hit "Ok". If you have a Dell for instance, there will be a bunch of Dell processes running in the background. Just about all that junk can go. A healthy computer on average runs about 25-40 processes.
Just like someone else said, run some spyware and antivirus scans. Download the following software and run them.
Spyware:
http://www.download.com/Ad-Aware-2007/3000-8022_4-10719977.html?tag=lst-0-2
http://www.download.com/Spybot-Search-Destroy/3000-8022_4-10401314.html?tag=lst-0-1
Antivirus:
http://www.download.com/AVG-Anti-Virus-Free-Edition/3000-2239_4-10703202.html?tag=lst-0-1
You need to run both Ad-Aware AND Spybot to get all the spyware on your PC.
If you are running Windows XP, min RAM should be 512mb. Windows Vista is about 3.5 times that.
Healthy computing!