Outlook Express isn't supported by Windows 7. The (roughly) equivalent program from Microsoft isn't even bundled with Windows 7. You have to download it (for free), as part of Windows Essential, from here:
http://windows.microsoft.com/en-us/windows-live/essentials
The easiest way to get your data from your XP computer to your new one is probably to install Thunderbird on both. Then, on the XP computer, follow these instructions:
http://en.flossmanuals.net/thunderbird/migrating/
Then use MozBackup to transfer the Thunderbird data on your XP computer to your new computer:
http://mozbackup.jasnapaka.com/
I forgot to give you the name of the Microsoft email client (that's bundled with Windows Essentials). It's Windows Live Mail. But I wouldn't recommend using it anyway!
I'd find it hard to recommend anything from Microsoft, Jordyboy! To be fair though, there's probably not a great deal wrong with Windows Live Mail but (for someone who has been used to Outlokk Express) it's probably less intuitive to use. [It also comes as part of a bundle of software, cluttering up people's computers with stuff that they might never need].