How do you completely wipe all your history off of a computer, like all history of you being on one?
I've had to use a relative's laptop for a while so I'm trying to get rid of anything that shows what I did on there but I'm not sure what to look for. Like past things I've had installed or downloaded, there's still files and indexes that are left over from me using when I used the computer, so is there a way to erase any trace of when I used the computer?
No way to just do it in one swoop, unless they had a backup of their computer before you used it (at least, no way that I know of).
Clear browser history from (time you started using) until (day you've stopped using) along with cookies, cache, etc..
Go through and delete your recent files - arrange the files by date. Don't mess with the other person's files. I'm sure you probably have a good idea of what you downloaded, created, etc...
That's all the advice I can think of off the top of my head. I'm pretty tired, so my apologies if it's not the most constructive advice.
@tath
That leaves all the data behind, so it's no good if the data is sensitive. If someone wanted it badly enough, they could very easily recover it. I've deleted partitions by mistake before and been able to recover a fully-working filesystem by simply making a new partition in exactly the same position as the old one. I've even done it intentionally as a faster way to resize a partition. If you format the partition, it does write over the old file tables, but it doesn't erase any other data, while defragmenting is only possible when there's a filesystem, and actually makes recovery easier, since much of the data will be contiguous.
The best thing to do is either the sledge hammer, magnet or a program which performs "secure deletion" (deletes a file and overwrites it a number of times) such as CCleaner.
then the best option is to write 0's in every sector ;)
On that note, just in case there is anyone here who hasn't heard of it, DBAN is the best route IMHO. Mostly for it's simplicity of use and pricing tiers: http://www.dban.org/