Is Dell precision 380 is compatible with dell 390

You will have to wipe the drive and re-install Windows on it after moving it to a different Dell model, because the existing Windows installation will not start on different hardware.

Other than that, there shouldn't be any problems with it.

Both 380 and 390 models look exactly the same and must have the same hardware with maybe a difference in processor speed or model. So the slight difference may not be enough to cause a non-bootable system... the only inconvenience may be the legallity of the swap, but rather tan moving the HDD from the 390 to the 380, I'd suggest replacing the failed 390 hardware with the one on the 380... or to use the Product Key on the COA sticker on the case you use so not to vioate the Microsoft COA Agreement

Here are the specfications for each 380 & 390 Model... the difference is CPU P4 800Mhz on the 380 and Dual Core 1066Mhz on the 390, so the difference is probably enough that it may take a repair install to make the sytem run, and the swap would be a downgrade from Dual Core 1066Mhz to Single core Pentium 4 800Mhz.

http://www.stikc.com/Catalog/Dell-Precision-380
http://www.stikc.com/Catalog/Dell-Precision-390