Although there are a few ways of spoofing your IP while on a network such as the internet there are always certain parts of yor computer that will always have the same identification mark. Such as your MAC (Media Access Control) Address. This is a mix of numbers and letters that are assigned to your network card, or your ADSL modem, or any device that communicates on a network. The MAC address can be modified with certain tools. Also there are things like your computer name, which can be changed easily. There are also other things such as your motherboard ID or serial number, which in some cases can be transmitted over a network, and to change these ID's would require a flash (rewrite) of your BIOS (Basic Input Output System).
So, even with an IP that changes every second via a proxy server, or if you change your MAC address and computer name each time, you will still need to flash your BIOS everytime, it can be done, but its a lot of hassle. If you are upto something you shouldntd be, then use someone elses computer, simple.
But remember if someone or something wants to track you then they will. Simple as that.
Cheers all
DoZZa