Computersight -

in general we have seen that a computer gets slow in a few months the main causes behind this are

1. You are running software that needs a lot of resources
2. The computer is copying files as well as too many files
3. The antivirus is checking files/processes for viruses.
4. You were switching to/from a program that requires a lot of memory (e.g. a game).
5. Some program has a memory leak and is allocating more and more memory.
6. A program you have loaded needs more RAM than you have available.
7. A program that works a lot with the hard drive is running, such as a disk defragmenter or a bittorrent downloader.
8. You are running too many programs at the same time.
9. New hardware you added is being detected.
10. If you are on a networked computer such as at a school or corporation then somebody with the right authority could be watching your computer if it suddenly gets slow and gets normal again later on and none of the above are the problems.
11. You enabled additional visual effects.

 but the most common is the fragmentation of files and many registry entries which are slowing down the performance of a computer

to fix this you can do many things but the most common one that i advice are:

1. to clean the missing registry entries by the help of registry cleaners
2. to defrag your computer once in a month.
3. checking for virus infections.

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