If it is is the Temp folder then it is probably left over from a programme or file that you have opened on your machine. You can delete it manually using Windows Explorer if you feel confident about that, leave it in the Recycle bin for a few days to see if it needs to go back (unlikely). Or you can download CCleaner click here and it should get rid of it for you as it will clean all your temp folders if asked to.
Yes it is possible that it could slow down your machine.
You might also want to try running AdAware click here and Spybot click here these are excellent tools for getting rid of unwanted items on your PC