When the PC freezes the workstations do the same because they are constantly talking to each other and updating files. Usually if the office PC freezes you will be able to at least have the option to go into stand alone mode. It can be:
1) A virus.
2)The CPU/processor chip or motherboard.
3)Bad power may be the culprit.
Have them replace the PC if you have a contract. If not you can try purchasing a used one on ebay but make sure that it is the exact same model.
Do you have a good line conditioner/UPS in place? Some people find it hard to believe but in my 25+ years in this business I have seen some crazy stuff happen due to bad power.
When I was with Micros, they made sure that every piece of hardware sold (on a new order) had a Power Var line conditioner that the hardware plugged into.
Office PC, Monitor, Switch Router - used a large unit that also had battery back up
Workstation and Thermal Printer shared a line conditioner with 1.29 amp
Kitchen printers each had a line conditioner with .69 amp
Good Luck