If you really think its slow then backup your stuff and re-format it. Then first thing uninstall everything you don’t want that came with it from Dell. Just leave Roxio and Quickset, the Dell webcam, you don’t even have to have Dell Support Center.
Then uninstall the anti-virus that comes with it, Norton now is it? Dunno. Get rid of it whatever it is. Get a copy of Kaspersky Internet Security, (download a fully working demo then buy if you want) you can get this cheap from scan.co.uk, just buy the oem version (get a 3 license version if you have other computers, its good value). Kaspersky is the best one on Vista for not slowing machines down, plus its not overly fussy to monitor what its doing for people who don’t know all the jargon that comes with software firewall rules etc.
Once you’ve got all that on there then just let vista do all its updates for god knows how many reboots!
Then go to dell.co.uk and install all the new drivers and bios stuff you can get your mitts on.
Don’t install software you don’t use, not even to try it then uninstall. Consider using Firefox for the internet.
It will run like a rocket if you do the above. I’ve got an XPS M1330, the one with the smaller screen, only got a 7500 cpu and 2Gb of Ram and it runs really fast, it never pisses me off one bit. Don’t bother wasting money on 64 bit Vista, if all you’re after from it is proper use of the last little bit of your Ram then forget it, it won’t be worth it in the slightest. Spend that money on beer instead.