Hi.
The system I think your talking about it what's called Dynamic IP (Internet Protocol) addressing. An IP address is the address of your connection to the internet - it's a unique address that only your computer has. This IP address is allocated to you when you dial up to the AOL server and will more often than not change every time you connect to the internet. Each ISP is allocated a certain amount of IP addresses, and they have to make the most of this allocation. You can get a static IP address if you want, but you'll either have to pay an awful lot of money or go Broadband (which isn't exactly cheap). As to making you untracable, not strictly true. Your ISP will keep records of when you log on and for how long etc, and can trace you through your username and hence what IP address you had or that particular session. This said they (no conspiracy theories please...) more than likely won't want to trace you, unless you partake in some dodgy activities.
The biggest issue with a static IP address over a Dynamic one is to do with hackers. The majority of dynamic address are for slowband 56k Modems, and to a hacker it's like stealing a Lada - slow and boring. They'll be after the good stuff - the Broadband type connections as these IP addresses are generally, but not always static as your always connected (my IP address does't change cos I use ADSL).
Spam is sent to your e-mail account, not to your computer, so a dynamic or indeed static IP address won't stop spam. A good e-mail filter is the order of the day!!
Hope this helps.
Cheers.