It's so slow and hangs constantly when opening even small files or programs. It's around 3 years old now so it's had a lot of work and junk installed over the years I guess
What's the best way to do a fresh install nowadays?
I don't have the original disc but I have the disc from when I upgraded to Mountain Leopard recently
Also any tips on doing it, things to avoid, good ways to backup data etc
I have an external HDD and also regular time machine backups
Thanks
You have two options.
1. Reboot holding down Command-R; this will boot you into the Mountain Lion recovery partition. Use disk utility to wipe/reformat your system volume/partition, and then you can reinstall Mountain Lion afresh. It'll prompt you for your AppleID to verify that you own Mountain Lion, and then once that check passes, your computer will download and install the OS on the spot. Get cozy, it'll be a while. When it finishes, you'll be ready for takeoff.
2. Go to the App Store, redownload Mountain Lion. Quit the installer app when it loads upon the completion of the download. Find the installer in your Applications folder. Right-click on it, and go to Show Contents. Then navigate to Shared Support. You should see a file in there called InstallESD.dmg. Copy that file elsewhere so you're not using the version in the app. Then from the newly created copy, right-click, and select open with, and then Disk Utility. Or just drag that file into Disk Utility. Or open Disk Utility and navigate to the newly copied InstallESD.dmg file. Either way, you get the idea. From Disk Utility, either:
A. burn InstallESD to a dual-layer DVD or:
B. take a thumb drive, no smaller than 8GB, and format it as Mac OS Extended (Journaled), and under Options, make sure it uses GUID and not Apple Partition Map or Master Boot Record. Once your drive is formatted and ready to roll, go under the Restore tab and under Source select the InstallESD image, and under the destination, select your thumb drive. If that fails, try mounting InstallESD.dmg and under Source, selecting Mac OS X Install ESD (as that sometimes is necessary).
Either way, this will leave you with a perfectly bootable 10.8.2 install media.