Forget about upgrading your existing computer and get a new one. Apart from anything else, it will be cheaper!
Assuming that your monitor is still OK, all you need is a refurbished Windows 7 computer from here:
http://www.accomputerwarehouse.com/products/computing/refurbished-computers
(NB: That's not just a random link obtained through a bit of googling. It's where I buy from, both for myself and for friends. It's an excellent company).
You can, of course, then upgrade to Windows 10 for free but I wouldn't bother. (I'm sticking with Windows 7).
If you've been using Outlook Express 6 on your old computer, you'll presumably want something similar on your new one. Windows 7 doesn't come with an email client but you can download Windows Live Mail (as part of Windows Essentials) for free:
http://windows.microsoft.com/en-gb/windows/essentials
Alternatively you could use a third-party freebie, such as the excellent Thunderbird:
http://windows.microsoft.com/en-gb/windows/essentials
You'll then need to export your data from Outlook Express to a memory stick (or other external media) and import it from there into Windows Live Mail (or Thunderbird):
http://www.cybernetman.com/kb/index.cfm/fuseaction/home.viewArticles/articleId/185
You'll also need to install all of the software that you had on your old PC onto your new one (or, of course, install something better) but we'll need to know what you require (e.g. DTP software, image-processing programs, etc) if we're to point you in the right direction.