Outlook Express is only available with XP. You may want to try Windows 7 as most Vista device drivers are compatable.
Dell Dimension E521 Support
I would suggst you create an Image Backup of XP before you try any upgrade.
Get drivers and downloads for your Dell Dimension E521. Download and install the latest drivers, firmware and software. Download and Update Dell Dimension E521 Drivers for your Windows XP, Vista. 440x 10/100 Integrated Controller, v.v4.60, A02 Broadcom 4401/B0 Ethernet.
EaseUS Todo Backup Home V6.0 http://www.todo-backup.com/
Free version: http://www.todo-backup.com/products/home/free-backup-software-features.htm
(Note: Install Todo and then create an 'Emergency Disk' before you start creating your first image backup)
Version 5.8 supports XP, Vista, Windows 7 and Windows 8
Keyless Install of Windows 7
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This will allow you to try Windows 7 for 30 days.
How to perform a 'Keyless Install' of Windows:
Boot from the Windows 7 DVD disc you created and start the installation process.
After you enter user name and password you will be asked to: 'Type your Windows product key', do not enter a key.
Also located just below the box where you would enter the key is a box with a check mark to 'Automatically activate Windows when I am online', remove the check mark and click 'Next'.
You will now have a 30 day grace period in which to activate Windows.
During this 30 day grace period you can try resolving your problems.
After you are satisfied all problems have been resovled then activate Windows.
Create your own Windows 7 installation DVD
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Download the appropriate Windows 7 .ISO file which includes Service Pack 1
Windows 7 Home Premium 32Bit: http://msft.digitalrivercontent.net/win/X17-58996.iso
Windows 7 Home Premium 64Bit: http://msft.digitalrivercontent.net/win/X17-58997.iso
After downloading the correct .iso file use ImgBurn or Gear ISO to create a bootable DVD.
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Create a Bootable Windows 7 DVD using the .ISO file
Burning a Windows 7 ISO File on a DVD
http://support.microsoft.com/default.aspx/kb/977640
ImgBurn - In addition to supporting the creation of Cds/DVDs from .ISO files,
it supports a wide range of other image file formats, and it's free.
(BIN, CUE, DI, DVD, GI, IMG, MDS, NRG, PDI and ISO)
http://www.imgburn.com/
Screen shots to help you use ImgBurn: http://www.imgburn.com/index.php?act=screenshots
Burning ISO Images with ImgBurn article: http://neosmart.net/wiki/display/G/Burning+ISO+Images+with+ImgBurn
(The steps are the same for Windows 7 or Windows 8), except you are not creating a Repair disc but a full installation DVD)
Note: Always use high quality DVD+R media and the slowest burn speed (4x or 6x) if offered a choice.
Device Drivers
Go to your computer manufacture's support web site and check for Windows 7 or Vista drivers for your specific model number.
If there are Windows drivers, then download to a folder on your hard drive and install all of them, starting with the Motherboard/chipset drivers, SATA, LAN, Audio, USB, Graphics, Webcam, Etc., and so on.
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List of Computer Manufacturer's support sites:
http://windows.microsoft.com/en-US/windows/help/contact-support/computer-manufacturers
Tips for fixing common driver problems
http://windows.microsoft.com/en-US/windows7/Tips-for-fixing-common-driver-problems
Vista: http://windows.microsoft.com/en-US/windows-vista/Tips-for-fixing-common-driver-problems
Graphics/Video drivers:
Check the Graphics card manufacture's download site for the most recent Windows 7 drivers for your card.
ATI: http://support.amd.com/us/gpudownload/Pages/index.aspx
NVIDIA: http://www.nvidia.com/Download/index5.aspx?lang=en-us
Dell Dimension E521 Motherboard Upgrade
J W Stuart: http://www.pagestart.com