I can't guarantee that this is correct but I think that I know what's happened and how to fix it.
At some stage when your computer was first set up (e.g. when Windows was installed or when you created a Windows profile), you (or whoever set up your system) entered your full name (or, at least, your surname).
When you set up an e-mail account on your system, Windows will have automatically filled in that same name as the name to send with your e-mail. If you created the account manually you could simply over-write this to change it to your choice of name. If the e-mail account was set up using software from your ISP, then the software will have done the same job for you.
Unfortunately, the Windows registry is prone to getting corrupted and, if your choice of name can't be found, then the system defaults to showing the original name.
So, how do you fix it? Like this: (I'm assuming that you're using Outlook Express 6 but it's much the same for other e-mail clients) -
Click 'Tools' / Click 'Accounts' / Click the 'Mail' tab / If you've only got one e-mail account then this should already be highlighted. Otherwise click on the relevant account to highlight it / Click on 'Properties' / If necessary, click on the 'General' tab / Look under User Information for the 'Name' and 'Organization' entries - these will show what information is being sent out with your e-mails / Change this information to whatever you want / Click 'Apply' / Click 'OK' / Click 'Close'.
I hope that this helps. (You'll know whether I'm thinking along the right lines when you check what is currently shown in the User Information section).
After all this, do a full virus scan to check that it's not a virus which caused the problem. (If you know how to do it, you might also consider running 'scanreg/fix' from your boot-up floppy, just to be on the safe side).
Chris