Don't. Even IF your computer can run Vista, why spend a couple of hundred dollars for Vista and additional memory to make your computer run much slower? Save your money; XP will be supported for two or three more years; after 2-3 years buy a new laptop, by then your computer will be 6 or 7 years old.
The D400 series is about 4 years old. I believe it has a 1.3 or 1.7 GHz processor.
For Vista you should have the following:
1-2GB RAM
Dual core processor or very fast single core (2.5+ GHz).
Support for DirectX 9 graphics with:
WDDM Driver
128 MB of graphics memory (minimum)
Pixel Shader 2.0 in hardware
32 bits per pixel
Run Vista upgrade Advisor.
http://www.microsoft.com/windows/products/windowsvista/buyorupgrade/upgradeadvisor.mspx
http://www.pcpitstop.com/vistaready/default.asp
Check to make sure Vista drivers are available for your printer, scanner, video card, sound card, etc.
Compare features of Vista versions
http://www.microsoft.com/canada/windows/products/windowsvista/editions/choose.mspx
Upgrade Paths from Previous Versions
http://www.microsoft.com/canada/windows/products/windowsvista/buyorupgrade/upgradepaths.mspx
XP Pro will not upgrade to Vista Home Premium
How to Install a Vista Upgrade on a Blank Hard Drive
http://gizmodo.com/gadgets/pcs/how-to-install-a-vista-upgrade-on-a-blank-hard-drive-232770.php
How to Clean Install Windows Vista with Upgrade Media
http://www.winsupersite.com/showcase/winvista_upgrade_clean.asp
Upgrade, Vista Home Premium, Students Version: $69
https://usm.channelonline.com/rtech/storesite/Products/Overview/?id=M003552199&fr=AFUNtQXAV5DJSTHyHoRdooiOmLbxqI-YexIDVLn-VWWb2z7s93jy_ZQAAAAAAAAAAA&gl=us&hl=en
http://www.campustech.com/c/campust/MSCD23142WIAE.htm?mv_pc=froogle&utm_source=froogle&utm_medium=ppc&utm_term=Windows_Vista_Home_Premium_Upgrade_&fr=AHdWIlrNhy9nFC0a7TDK7_XvuDbMwHCIpC1el1piQ6m6AAAAAAAAAAA&gl=us&hl=en