Pretty much non responsive after close to 4.5 years... Any suggestions?

Question

I've owned by PowerBook G4 since Aug 2004, and it has served me pretty well. I had a battery change since my battery was one of the recalled ones. It was working fine up until about last month, when I started to notice that it was taking unusually long amount of time to execute simple tasks such as opening Safari, or playing videos.
I know that I cannot expect top notch performance since it's close to 4.5 years old, but if its taking me close to 10-15 minutes just to boot up, enter my password to log in, open up Safari, go to Google and search for something, there is clearly something wrong. I backed up most of my music, video, photos and documents to an external drive recently and tried to clear out much of my hard disk as possible and it showed a little improvement, but about 2 weeks ago, it started being cranky again. When I connect my iPod to load songs, it takes forever for it to recognize, only for it to freeze. First the cursor doesn't respond, then it becomes the rainbow circle, then just stops.
Anyone have any clue as to what could be causing this? Would it help if I back up the entire disk and reformat the hard disk?
Also the new batteries aren't working out too well, if I don't leave it connected to the power supply, the battery doesn't last beyond 15 minutes before it goes off to sleep mode.

Answer 1

I realize you wrote that a friend has already helped you. Here are step by step directions.
First, you need to format the new HDD
Next, install OS X.
If you need to migrate information from you old drive to the new, please post back and ask.
Formatting, Partitioning Erasing a Hard Disk Drive
• With computer shut down insert install disk in optical drive.
• Hit Power button and immediately after chime hold down the "C" key.
• Select language
• Go to the Utilities menu (Tiger) Installer menu (Panther & earlier) and launch Disk Utility.
• Select your HDD (manufacturer ID) in left side bar.
• Select Partition tab in main panel. (You are about to create a single partition volume.)
• Click on Options button
• Select Apple Partition Map (PPC Macs) or GUID Partition Table (Intel Macs)
• Click OK
• Select number of partition in pull-down menu above Volume diagram.
(Note 1: One partition is normally preferable for an internal HDD.)
• Type in name in Name field (usually Macintosh HD)
• Select Volume Format as Mac OS Extended (Journaled)
• Click Partition button at bottom of panel.
• Select Erase tab
• Select the sub-volume (indented) under Manufacturer ID (usually Macintosh HD).
• Check to be sure your Volume Name and Volume Format are correct.
• Click Erase button
• Quit Disk Utility.
Installation Process
• With computer shut down insert install disk in optical drive.
• Hit Power button and immediately after chime hold down the "C" key.
• Select language
• Open Installer and begin installation.
• Select installation option
• Choose to Customize and deselect Foreign Language Translations and Additional Printer drivers.
Optional: Check box to install X11 (Tiger and later) BSD Subsystems (Panther & earlier).
• Proceed with installation.
• After installation computer will restart for setup.
• After setup, reboot computer.
• Go to Applications > Utilities > Disk Utility.
• Select your HDD (manufacturer ID) in left side bar.
• Select First Aid in main panel.
• Click Repair Disk Permissions.
• Connect to Internet.
• Download and install Mac OS X 10.5.5 Combo Update (Leopard) or
Mac OS X 10.4.11 Combo update (PPC) (Tiger) or
10.3.9 Combo Update (Panther)
Computer will restart.
• Repair Disk Permissions as previously.
• Go to Apple Menu > Software Update.
• Install all updates.
Computer may restart after updates.
• Go to Applications > Utilities > Disk Utility.
• Select your HDD (manufacturer ID) in left side bar.
• Select First Aid in main panel.
• Click Repair Disk Permissions.
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