When you upgrade to DD-WRT on your WNR3500L, you'll find that the new firmware requires a file that ends with the BIN extension, if you ever want to upgrade or change your firmware from that point.
So, if you decide, for example, to revert back to the original NETGEAR firmware - you can't because that firmware has a filename that ends in CHK.
As such, this makes it harder to change back. One way to do it is to use a serial console, but if you don't have one... here's a way to accomplish it in Windows without one.
Fortunately, included in the DD-WRT firmware is an erase utility that will wipe the firmware and allow you to upload a new one.
Here is how it's done.
You first have to Telnet into your router.
To do this simply open up a CMD window and type Telnet 192.168.1.1 (or the location of your router)
You will then need to type in the name and password for your router. If you have not changed it form the original setup, then they would be Name:admin and Password:password.
Once in, you will be shown a prompt that you can use to erase the old firmware.
Type in "erase linux" and it will run and show you its progress.
Once complete, it will again show you a command prompt.
Type in "reboot" and you should see "Connection to host lost"
You can then close this window.
Now we need to put the new firmware on.
Wait about 5 minutes for the router to reboot and then open up a new CMD window.
You will then use TFTP to put the new firmware on the router. This program comes with Windows XP. If you are using Vista, you need to install it.
1. Open the Control Panel and select the Programs entry.
2. From the Programs and Features section, select Turn Windows features on or off.
3. Scroll down the list and place a check in the box next to TFTP Client. Click OK.
4. Give Windows a moment or three to install the tftp client. When its done, exit out of the Control Panel.
Open up a CMD window and type in "tftp -i 192.168.1.1 PUT NameOfFirmware.CHK" as shown below.
Wait a few minutes and you should see "Transfer Successful"
At this point all you need to do is wait. The router will install the firmware and reboot automatically. This could take 5 to 10 minutes.
Once the router is finished booting you can go back into your browser and setup your desired configuration.