Check one by one:
Before doing anything u must take all necessary data backups else it may cause serious problems.
1. The particular computer may affect by virus
2. It is possible one or more service(s) may stop suddenly
3. Corrupt any system related files
4. All of a sudden other 3rd party software may creates problems
5. Also check LAN card or Ethernet card for fault detecting
First u have to disconnect the particular computer from ur network.
Now check one by one (as told earlier), u may use in-built system checker tools or licensed utility software to find out the problems.