Need to have - include "modedefs.bas" - near the start of the program for serial to work.

Get rid of Sample after NEXT.

IF ftemp > 85 THEN set_alarm change to
IF ftemp > 85 THEN GOSUB set_alarm (look in the manual)

Look in the manual for how SERIN works.

Take things piece by piece. Try reading the temperature to hyper-terminal first to make sure that is working, the add more functions after that.