Where is the hard code speed found?
Any Windows version;
- Device Manager
- Network Card/Interface
- Properties
Dig into the various driver settings. You should find a setting somewhere that’ll afford opportunity to set from AUTO to 100Mb.
If Windows doesn’t ask to reboot AND you speed does not improve. Reboot both boxes anyway.
Other thoughts;
1) Physical Interface. You’ve indicated that you’re running a CAT 5 cable. Are all the pins terminated? Are the terminations clean? Split pairs? Have you swapped cables just for grins?
2) Traffic. Anything else running on your network? Some device spewing a bunch of packets? Wired or wireless, is something is crushing your router?
3) Common Point. When was you router/switch last updated and restarted?
A 100 foot patch cord would work fine. You need to cross TX and RX (4 and 5 I think). No distance issue inside of 300 - 330 feet.
Good luck.