If I understand correctly, then you're options are
1) change the start in folder of sqlplus to the location of the script
or (I prefer this next one)
2) set the value of SQLPATH in regedit to the script folder. SQLPATH can actually contain multiple folders, so you can spread your code into multiple locations (in some logical order). SQLPATH might already be defined under ORACLE, ALL_HOMES or HOME# - if it is, modify it there. If not, define it in one of those keys and see if it works (the key location can change on some versions of sqlplus/windows, and defining it at the wrong place can make sqlplus fail to start). To make SQLPATH contain multiple folders, delimit the folder names with a semicolon (and don't put any extra spaces at the beginning or end of the folder names).