So I have an old hard drive with windows xp and personal files on it. I want to wipe it clean and put windows 7 on it and us it as a secondary computer. What is the best way of doing this? Also I was told once something is put on a hard drive it can never be erased, is that true? If it is what would you suggest for just removing the data and restoring windows so i can have it run clean and fast as possible.
depending on the make and model you may be able to get a low level format for it. this will return the drive to a completely raw state. if not then use somthing ccleaner that will enable you to wipe the drive multiple times... or look here http://www.dban.org/
if you are keeping possession of the drive just format it during the windows 7 set up--much quicker--securely deleting it takes a long time if you do it to above dod standard--if you ever give it away or ebay it etc use the program hexit said and do 7-10 overwrites--can take all night long depending how big the drive is
since you are reusing for yourself, just format the drive during windows 7 installation as already suggested. The install will detect all the drives and partitions available so select this drive and click the "Format" button.
If you have more than 1 partition on the drive I suggest deleting them until you just have 1 large "unallocated" space and then create a new partition followed by a format, do a full format instead of a quick when prompted to select one.