It won't delete everything, you just won't be able to put new stuff onto it without erasing those categories from it. For instance, you can keep the photos and music it already has on it, while syncing new podcasts to it. Each tab in the iPhone will have to be specifically "checked" to enable syncing that kind of data from that computer, and it will warn you "This will remove the current podcasts from your iPhone. Do you really want to do that?"
If you can somehow get the data off of that old computer, do so. You will be able to transfer the iTunes library with a little work.
If the old computer was also a Mac, then it's probably about as easy as it will get.
Make sure on your new Mac to use the same username (short name; the one you see as your home folder in the Finder) as you did with your old Mac, and grab the whole Music folder from /Users/yourusername and drop it in the same place on your new computer. (Shove the existing Music folder under your home folder out of the way with a quick rename). When you start up iTunes, it may update the library if the new iTunes is a newer version than the one you were using. (In fact, just go ahead and do software update to get iTunes up to date before you do this, because an older version of iTunes can't cope with newer libraries)
If you changed your username to be different on the new Mac than it was on the old Mac, you'll need to edit the iTunes Music Library.xml file under Music/iTunes and replace any occurrence of your old username with your new one before starting iTunes.
If your old computer was on Windows, then the process gets a little trickier depending on whether it was XP or Vista, but you'll have all of your songs and apps and such. You'll be editing one file and renaming another.
In both cases, make sure to use the same username on your new Mac as you used on Windows. Otherwise, you'll need to do more editing of the file ahead.
Grab your My Music or Music folder, and put it under your home folder. Rename it if needed to Music
Now you'll need to rename iTunes Library.itdb to just iTunes Library with no extension.
Then right-click the iTunes Music Library.xml file and open with TextEdit. This is where you'll need to replace some path names. This is where it gets a little hazy.
By default, in XP, the path names will start with something like file://localhost/C:/Documents%20and%20Settings/fury/Music/iTunes/iTunes%20Music/
By default, in Vista, the path names will start with something like file://localhost/C:/Users/fury/Music/iTunes/iTunes%20Music/
If XP, you'll need to use the text editor to find and replace the string C:/Documents%20and%20Settings with Users
If Vista, you'll simply replace C:/ with nothing - a blank string in the replacement box.
If you changed usernames as well, you will need to replace the /username/ string with /newusername/
So, if you changed your username from Maaz on the old one to MaazRocks on the new one (or changed any capital letters), you'll need to do a second replace: /Maaz/ to /MaazRocks/
Be careful about accidental extra spaces or slashes. You'll want the path names to end up beginning with something like this: file://localhost/Users/fury/Music/iTunes/iTunes%20Music/
Just to be sure, use Finder to navigate to the final pathnames you see in that file before saving, and make sure it's a valid path; e.g.: click on Macintosh HD for the root of your drive, then open Users, then open the folder with your username, then open the Music folder, then the iTunes folder, then the iTunes Music folder. If you see all of your stuff there, then you're good to go. Save the file and start up iTunes, and after it thinks for a few minutes and updates your library, you will have successfuly transplanted your iTunes library from Windows to Mac.
In any case, from Mac to Mac or Windows to Mac, you are not going to transfer any old backups with these instructions, but iTunes will make you a NEW backup once you sync after opening iTunes with the transplanted library.
If, unfortunately, you can't retrieve the data from the old computer, you can at least have the iPhone transfer purchased songs and apps to the new computer. When it's hooked up, right-click the iPhone in the sidebar on the left, and click "Transfer Purchases"
Thanks for the post man. But all my music and everything else Is gone. From my comp. What Iam trying to do now is use "iPhone browser" to get all the data off my iPhone and save it somewhere else. Yet the program isn't working and I always get an error when trying to open it.
Ouch. That sucks. I recently killed a hard drive that had my entire music archive on it (along with my life's work, and a bunch of movies...) - the stuff I had on my Mac was just a small fraction due to the small hard drive.
I haven't ever tried to get anything off of an iPhone, beyond the apps & purchased songs... Good luck with that