check trace and log files (under %oraclehome%\rdbms73(?)\trace and log I believe).
Try to start database with svrmgr(73 may be svrmgr30 or 23, check in bin\) and see logs/trace for errors.

To start DB with svrmgrl:

set ORACLE_SID=<your sid>
svrmgr30
connect internal/oracle(or other password, default is oracle)
startup

Other 'low level' commands in execution order:

startup nomount
alter database mount
alter database open

and see what is going on. May be have to reinstall oracle binaries, configure registry, fix OS or hardware...