That system is 8 years old and considering that you may have the original HDD installed and the maximum RAM is 4GB, it is easy to believe that the system is sluggish. You can install an SSD that will help with performance but the system is being asked to perform at beyond it's maximum limit.

First thing first is try replace cmos battery with a brand new cr2032. Then clear cmos settings via motherboard RTCRST  jumper.  After that  disconnect hdd from pc and press F2 to enter bios and test for system responsiveness to keyboard.  If all good, do a Dell diagnostic test by press F12 and choose diagnostic after power up.

The 280/380 model is 18 years old and is considered obsolete now. 

Dell Model Years — Engineering Computer Network (purdue.edu)

One thing I differ on - One doesn't need to disconnect HDD to go into BIOS (and check keyboard responsiveness) or choosing Diagnostics.  I'd leave HDD connected when running Diagnostics in case HDD is failing.

Experience I had with 380 with Win10 on it is that it was hesitant just for the internet.