|Java is not often needed any longer so uninstalling it is usually safe. If you find that you need it, it is easy to reinstall.|
That said, I doubt that Java is the cause of slowing down your computer.
First, uninstall any program that you never use, Google it if you are not sure what it is for.
Second, Run Disk Clean Up on your hard drive(s). Check everything to remove and OK it, it may take a while if there is a lot of junk. (Open Computer, Right Click on your C drive and select Properties. Then click on Disk Clean up)
Third, look in Task Manager to see what is running in the background. You will often see programs running that you use only occasionally. At this time, End Task on these later you can see of the program offers an option to not start with Windows but only start when you want it running. This is done because every programmer wants you to be able to launch their program instantly but when there are many programs running in the background, the plan backfires since all of these bog down the system instead of allowing quicker starts for a select program or two.
Finally, if you are running certain antivirus programs, especially the full suites, and especially Norton and McAfee, they are using more system resources than necessary to do their job and can slow your system as well. I recommend running Windows Firewall (not others) and a more efficient antivirus program like Avast Free Version or Webroot Secure Anywhere (pay for, but my favorite), the antivirus only versions, not the full suites.
You have to be a little bit crazy to keep you from going insane.