Deleting folders after Oracle upgrade I upgraded from Oracle 10g to Oracle 11g in Windows Server 2003.
New Oracle home : C:\app\oracle\product\11.2.0\db_1
Old Oracle 10g was installed on: C:\oracle\product\10.2.0\
Questions

Is the 10g folder is still useful?
Could I delete the 10g folder without a problem?
Old oracle home is no more useful after upgrade.
To safe remove it you should do either:

Start &gt; Installation Products &gt; Oracle Universal Installer. 


from here you can select deinstall and remove the old oracle home.
One alternative is to manually perform the process running the command:

setup.exe -silent -detachHome ORACLE_HOME="C:\product\DBHome" ORACLE_HOME_NAME="DBHome"


You have to replace ORACLE_HOME_NAME and ORACLE_HOME with the one on your system. To find them, in Windows, first get the value of 

\\HKEY_LOCAL_MACHINE\\Software\Oracle\inst_loc. 


in your windows registry. Then navigate in the inventory location and open the file:

inventory.xml


here you'll find some lines like:

&lt;HOME NAME="OUIHome1" LOC="C:\oracle\product\10.2.0\" TYPE="O" IDX="4" /&gt; 


This is your line. Location is the one you know, write down the name and perform the command. When oracle home is detached you can manually delete it from windows explorer.

IN addition, manual delete without detaching does not cause any problem but leave some annoying pointer in the inventory