Windows 7 has some built in repair options
***System Recovery Options in Windows 7***
# Restart your computer.
# Press/tap F8 ***''before''*** the Windows 7 logo appears.
# At the Advanced Boot Options menu, select the Repair your computer option.
# Press Enter.
# System Recovery Options should now be available.
Here are some more detailed instructions;
https://www.bleepingcomputer.com/tutorials/start-the-windows-7-recovery-environment
That is quite an old computer. Have you checked if the fan is spinning and there is not too much dust on everything? It could be over heating.
Here is the repair guide:
[[Topic:Dell Optiplex 745 Small Form Factor]]
Just ask if you have any questions.