Partly the answer depends on what you want. If you want a new machine you can get a new one for $300-$400. There's not much point in spending more.
If you get Win7/8 your likely choices for email are basically Thunderbird (designed to look like OE but can be a bit buggy) or Windows Mail, which can be downloaded. TBird can import your old OE email, if you save the folder with the DBX files in it.
If you can't get a Win7 box and don't want Win8 then you can buy an "OEM" DVD to install Win7 on the new box. If you don't mind Win8 then just watch out for Microsoft trying to get you to sign up for online services at every turn.
Another option is to save your current computer. The hard disk is what usually goes. Most of the other parts can last a lot longer. But here it gets tricky. Newer hard disks are SATA interface. Yours is almost certainly IDE. (The flat ribbon cable.) You can replace the hard disk cheap but you'd need to either a) find an IDE replacement or 2) buy an SATA PCI card to plug a new SATA drive into. This probably sounds very confusing. It's actually very easy *if* you know the details. You might want to find a friend to help. The basic idea:
- Install a 2nd hard disk temporarily.
- Use a disk management program to copy the old disk to the new disk. (I like BootIt. There's also a freebie called Macrium.)
- Set the new disk as the only disk, removing the old one.
- Drive your updated PC for another few years, if you're lucky.
(There are details about plugging in disks, etc. but it's too much to get into here.)