Some Anti virus programs are pretty intensive, I like NOD 32 from ESET. Uses very little of your CPU and very little RAM to run, plus it is a great program. I do not use windows built in firewall, it is terrible and does not stop much beyond Malware. COMODO firewall is what we use and it is free.
Uninstall junkware is a great thing to do also, as mentioned.
There are tons of articles on the net about speeding up Vista. Though be careful as some source may be a bit dubious. Stay away from software that claims to speed up your system.
I have some great articles on speeding up the Vista, but as said, most of it comes down to your OS and hardware specs. I use 4Gigs or RAM ( Mushkin high end DDR2 800 ) and 64 bit Vista Ultimate. I use alot of After Effects CS4 and need 8 Gigs for those heavy apps. ( ordering the other 4 gigs very soon ).
To go from 2.4GHZ to over 3GHZ is called overclocking and can kill your CPU if not carried out correctly. You need to enter the BIOS to overclock, so research on your particular motherboard is vital. Most OEM;s ( Dell, HP,Compaq so forth ) can not be overclocked. Plus if you did overclock an OEM, you void any warranty from that company. If you get into overclocking, you need after market heatsinks / fans as stock fans are not designed for anything beyond the default settings of that CPU.
Windows Vista is set to defrag automatically each week or whatever you set it to. Windows Defrag is very slow, so some advise a third party software for defragging.
For ultimate performance, you need to get a RAID 0 set up with 10K - 15K rpm Hard drives or even SSD drives. ( Still a great debate out on SSD drive ).
Overclocked RAM, Quad core ( Intel have best right now ). Overclocking is fun but be careful.
I gotta go to bed now, but will add some info that I got 2morrow.