Hello
You can acheive this either by creating the limited account or enable the
guest account.
1.creating the limited account:
start(button)->settings->control panel->user accounts.From the screen select
"create a new account".Type a name for that account and hit next.In the next
screen select the account type as "limited" and hit create account.
2.Enable guest account:settings->control panel->user accounts.From the screen
(of users),click on the Guest account then in next screen "turn on the guest
account".
Now you can log with guest(password empty) or with the account you have
created in 1.So users who log with these two accounts cannot install the
softwares.
Hope this helps..