Vista - windows firewall

i have been using windows vista home premium 32bit.i had changed the firewall settings to off by mistake.right now i was unable to change the firewall settings as it says the service is unable to start,and even it asks for the administrator permission although i have been using the system as an administrator.in the event system log it displays the problem as a dhcp error.i was unable to connect to internet through my broadband connection.i could not access to the restore option even.when i wanna restore the system it displays that it is currently working in restore point.
i was unable to understand the problem and how to clear it.i am in desperate need because i could not access to the net.
the following problems i even face with my system
1.it takes around 10 minutes to log into the account
2.i didnt uninstall the avast antivirus buy yet its not working and even couldnt find it and i tried to install it but its not working
3.i could not log in via another user account.
4.it fails to access" system notification service".

i would be grateful if i get the suggestion as soon as possible

Namaste, bhanukishan, and welcome to the forums.

It could be any of the above, but the 10 minute boot seems to indicate virus infection first and foremost, especially as Avast has stopped working as well.

do i need to i re install windows vista?

No, I am pretty sure this is solve-able without a full reinstall. However, we don't ahve any tutorials here about Windows Firewall.

Have you checked the Vista Security center to see if you can enable it from there?