I assume by renovate, you mean upgrade? You are intending on replacing the MOBO with a newer/faster one? Unless it is a Dell, you can do that easily. Get the size of the MOBO in inches, it will tell you if it is a MATX or a regular ATX footprint, then buy that size. Generic cases, as well as Compaq, HP E-Machine and other branded cases are fairly easy to swap MOBO's in. Some cases have funny footprints for power supplies, plus many are really small. This means you are likely to need a new power supply too. If you are really intent on upgrading easily, your best bet is to get a case from Mwave and a combo board from Newegg. Then you can migrate your old components if they are still usable. This limits your options somewhat, but if you really want to make this a do-it-yourself project, it is fairly straight forward. For more exotic hardware, like blindingly fast systems for gaming, you can go to them and have the basics built to your specs then add additional components.

For just normal computing, I sell the s**t out of this combo:

Case: www.mwave.com/mwave/viewspec.hmx

MOBO: www.newegg.com/Product/Product.asp