First off DVD Rom drives are simple to replace, the hardest part is usually removing the desktop face plate & opening up the case. Unplug the SATA cable and the power cable from the back of the DVD-Rom. Next, if it has rails then you slide it out, otherwise just remove the screws and slide it out. If it has rails move them to the new drive. Slide it in and screw it in (or make sure it locks in place for rail system). Reattach the SATA cable and the power cable to the back.
Before you remove a hard drive you should back up your data & you also want to make sure you have your Windows Product Key - if it is not on a label or with the W7 DVD then it is possible to get it from the OS while it is still working (easiest way I know to get the key is with freeware software SIW).
Physically replacing the hard drive is usually just as simple as the DVD-Rom. However, if the drive is still good, you have the space inside (an extra bay), an available SATA connection from the motherboard, and a power connector from the power supply then you may want to consider keeping the old drive and just adding the new one as a secondary.
Do you plan to clone the old drive or perform a fresh install? If doing a fresh install then simply pop it out and put in the other one, put the case back together, boot up with the W7 DVD in the DVD-Rom drive and install.