Temp files are like post-it notes your PC makes when doing certain things like installing/uninstalling programs, etc. When the process is finished they're useless and should get deleted but sometimes they're not due to small errors or bad coding from the program's author. All they do from that point on is take up unnecessary space and occasionally cause errors in the program itself. There is absolutely no danger in deleting them. In fact, if you go to Start > All Programs > Accessories > System Tools > Disk Cleanup, you will see that windows actually has a program to delete them.
Between temp windows files and temp internet files you could easily have over a gigabyte of useless junk sitting around taking up space, like a desk covered in crumpled notes when you finished your essay months ago!
(Sorry for the lame comparison, some people prefer to liken this stuff to real life things

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