as MrBounty says Vista need lot of ram, so if it hass less than one Gig then upgrade.
Probably a good idea to run a full spyware scan on it to if it's not been done lately.
http://www.download.com/Ad-Aware-2008/3000-802 2_4-10045910.html?tag=mncol
Also Vista can be sped up a fair bit on some systems by turning off all the fancy visual effects. Right click on Computer, select "Properties" then chose "Advanced System Settings", and, on the "Advanced" tab, click the "Settings" button under the "Performance" section. Go to the "Visual Effects" tab; there you can choose options to enable or disable certain visual effects,
or just click the "adjust for best performance" button at the top
if it's still slow then it's possibly loading a load of rubbish at startup thats not needed