answer:When you open up iTunes on a new computer, you can transfer all your purchases to the new library by using a relatively newish feature called iCloud. This transfers every purchase you’ve ever made from the account, when previously you could only transfer what was on your iPod at the time. As a side note, the last time I checked you could not transfer movies that you’ve bought with your account, only music and apps. However, this only transfers purchases from that account. If you’ve put music from CDs you own or anything like that on your iPod, those won’t be transferred and you’ll have to put them back in your library. Another side note: One iTunes account can only be authorized on five different computers. If you want to authorize it on a new computer and you’ve already done it with five others, you’ll have to deauthorize it on an old computer. If you don’t have the computer anymore, you’re pretty much out of luck. Basically, don’t download iTunes on more computers than is necessary, and always deauthorize it on a computer that you won’t be using it on anymore. As for answering your second question, from prior experience, just having iTunes installed won’t slow your computer down much, if at all. While iTunes is pretty slow when it’s in use (much less so on newer computers), I never experienced a significant change in computer speed just because by downloading iTunes. Hope this was helpful!