This is never going to be a fast machine
Dual core at 1.8 ghz is potentially slower than a 4 year old single core pentium on a desktop.
Add vista into the mix which is more of a resource hog to start with (1 gb of RAM to run smoothly and more cpu cycles than XP just to have the desktop up and running)
Your best bet is going to be cutting as many system resources as you can while safely keeping the system stable. Start with obvious stuff like print spooler, remote access, windows updates (do these manually once a month), Lightscribe (if installed, Messenger services (if installed), uniteruptable power supply, etc etc etc.
Remove all of the junk ware that comes pre installed unless you are using it get something like REVO UNINSTALLER to completely remove all of the leave behinds and registry entrys that the Control pannel add and remove software leaves behind
Defrag once a month and don't allow your hard drive to fill up with clutter
Upgrade to windows 7 when available in 2010
However even with all of the above do not expect a lightning fast machine