I am making a new pc with new motherboard, processor, ram and windows 7. I am using, however, an old pc case power supply and hard drive, the hard drive has windows xp on it and all the files i want, i have backed it up so dont mind loosing the files but should i wipe it fully and then load it as usual with the other hardware pieces and then put windows 7 on or do i need to do anything else. this is my first build so im quite cautious of compatibility of hardware.
You have a couple of options. The simplest is doing very little. Install it as normal. When you go to Install Win 7 from the CD, the first thing to look for is the option to Delete any and all existing Partitions on the HDD you have in the machine. AFTER that you do a normal Install. This will completely destroy all old data.
There is a extra caution you could do if you choose. Modern HDD's (anything SATA, for sure, and some before that) do a bunch of background work on their own that your OS never knows about. For example, every time a sector is read, the HDD examines the signals coming from the head and decides whether they are clean and strong, or weak. If they are weak, it will re-read to be sure it got the data right. Then it marks that sector off in its own internal files as a "bad" one never to use again, and replaces it with a good one from a secret supply of spares it has from its first Low-Level Format in the factory. Lastly, it writes that recovered data to the new good sector and you do NOT lose anything. This is NOT the same as Windows' CheckDisk utility - Windows never knows about all this and does NOT know about this now-hidden "bad sector".
In your case, since you have a complete backup and do not want to save any data on the old HDD, you can force this process. Get the set of HDD Diagnostic Utilities from your HDD maker's website. In them look for a tool to Zero Fill your old HDD. Be VERY SURE you only do this on the OLD one you want to wipe out! When you do the Zero Fill, every sector that is filled (and that will be ALL of them) will also be read back to verify a good write, and that also triggers the internal signal quality check. This means that EVERY sector on the HDD will be tested, and EVERY "bad" sector will re replaced with a good one, so it has NO "Bad Sectors" for Windows to have problems with. This process will take MANY hours to go through the entire disk. But when it's done, you have a HDD in perfect condition and totally empty, just like a new one.
Of course, this process of substituting good sectors for bad ones has a limit. As it is done, the HDD also keeps track of how often this has happened, and how many more good spares it still has. Eventually if the spares list gets low, it warns you via the SMART diagnostics process, and that's when you arrange to buy a new replacement unit and clone all your stuff to it BEFORE the old one develops a problem it cannot solve and you risk losing some data.