Yeah, don't use the CODE tag, just copy and paste the link so we can CLICK it.  Now we have to copy and paste it into the URL bar, I ain't doin' that...

ADVISE:
You really should have 2GB RAM for Vista, more if can afford it.
Get a PCIe Graphics Card (I just got one for 19.99 with 256MB DDR2)

Upgrade the HDD when you can afford to, newer ones are always faster.  They also have 10K RPM Gaming Hard Drives that are REALLY fast.

Put as much RAM as the PC can handle, and force VISTA to use RAM instead of using the HDD as virtual RAM.  There are tweeks for this.  I only have 1GB of ram for Vista Ultimate, and it runs OK, but not like it can.  I get that PCIe graphics card today, so that will free up the 128Mb of shared memory to be put back into the O/S, and will help.    DDR2 ram is cheaper than DDR, I can get DDR2 1GB sticks for under 16.00 each at times, so maybe you want 4GB of ram...   I think Vista 32bit can only see 3GB, but here is a registry tweak for that too.   Vista64 will see 4GB+.

You can always upgrade the CPU.  Whatever SOCKET you have, like Core 2 Duo uses either Socket 478, which is still Pentium 4 class, or LGA775. The number represents the pins the CPU has.

All depends what You can afford, however, once you get up to like 300.00 in upgrades?  You can buy a new tower that is better.    Circuit City is going, and they have Athlon X2 towers for just over 330.00 right now, so I am waiting, they have a display case full of them.  If they go just under 300.00, I will probably pick one up.  They are 4500Ghz+ dual core systems... 320Gb HDD, 3GB RAM, and all that stuff...  Not a bad TOWER only deal..

IF you go to www.Justdeals.com or www.pacificgeek.com - You can get cheaper, MUCH cheaper 512MB PCIe graphics cards.  They have 1GB ones too, they get pricey up at 512MB+...

Well, if price isn't an issue, then buy a new tower...

if price isnt an option then a new computer all together might be a good choice though not always cheaper choice. but even if you continue to upgrade that computer over the years soon or later you will need to upgrade that mother board so my sugestion just start from scratch and build a new computer with one of those fancy I7 processorsif your an intel guy. you can always just transfer your old HDD so you will still have that old computer feel.