Hi
Try doing this- this method should stop users from installing softwares in Win Xp.
1- Type gpedit.msc in RUN
2- Goto Computer Configuration - > Administrative Template -> Windows Components -> Windows Installer
3- On the right hand side window, right click on "disable windows installer" and select properties
4- In the properties window, select "Enabled" then at the bottom , select "Always"
Hope it helps
Qi