If you can get into the BIOS (usually pressing Delete key repeatedly while booting), you can then change it to boot from Disc, from boot from c: and then use your CD and see if that helps.
Also. Vista Home Premium, cant see more than about 3Gb of RAM anyway, even if you tried to install 8Gb. XP Liked to have 2Gb of RAM (even if it was the older, slower PC3200 etc RAM) so actually, Vista isn't quite as RAM dependent as people make out.
Does like 3Gb though.
You need Vista 64 if you want much more RAM, but the driver issues with Vista 64, are worse that the 32 bit variety.
Vista rocks.
XP also rocks, but in it's own way.