Can I use my old HDD on my new computer?

So I am getting a new computer and I am wondering if I will be able to use the hard drive of my old computer on the new computer im having to save some money. But aside from reformatting the hard drive, is it possible if I just like use it as it is and just install the drivers of the mobo and gpu and everything else as is?

What im worried about is that if i use my old hard drive the drivers would like have problems with compatibility... and I would really like to use my old hard drive in my new computer cause it has a lot of files that i need and that are important as well...

If anyone knows the answer to my question I would greatly appreciate it!

Thanks in advance!

The first thing you have to worry about its if your HD is IDE or SATA and if it will fits with the new motherboard (if you have a SATA hd then you probably will not have any problem, but it is possible that your new motherboard doesn't support IDE).

For what concern the software, you must format it (i always formatted my hard drives before changing PC, never even tried to mount a HD with a OS already installed, i think it's a matter of habit).

Most, if not all mobos don't support ide anymore. Its a waste of space really. He can always get a usb/ide to sata adapter so he can back up all his stuff on to the new storage.
Also you don't necessarily have to format your hard drive when you switch to the new parts. I switch components all the time like cpu, ram and gpu. Though going from a quad core to a dual core is something I've never done so don't take my word for it.

Provided it's sata, it should be fine.
I used my old sata 70gb hdd from my old computer, went from a dual core, 512mb graphics, 2gb ram, amd athlon2+ mobo to a quad core, am3+ mobo, 4gb ram, 2gb graphics, etc.
It'll probably crack the sads at the start, but it should work.
I booted mine up (didn't change anything, still had windows xp on it) and it said 'the hardware has been changed' or something along those lines. It worked perfectly. albeit slow as shit.

Yes, you can use your old HDD in your new computer, SATA versions are backwards compatible and even if your drive uses IDE you can get a cable to convert the signal. Just a fair-warning that your read/write speeds will be limited to IDE read/write speeds, so things like copying files, loading game environments, extracting archives will still be slow, as if you were using your old computer. You can tell whether or not your drive is IDE by looking at the cables used to connect it to the motherboard. If it's a small, often red, cable, it's SATA, if it's a large ribbon, often a beige color or shade of grey, it's IDE.

If it's just a matter of some important files, you could buy a hard drive enclosure. Or better yet, you could get a Solid State drive to boot from and you can delete the os from you current drive to free up more storage space. Of course, if you want to save every bit you can then everyone else is correct; you can just plug in your old drive and let 'er rip.