Chris, as you have a humble little PC, I’ll assume you are a Windows user. The .tar format files are not compressed as such, they simply contain a group of files (if I remember correctly TAR stands for Tape ARchive). Use WinZip or WinRAR to open them up. The images are in MrSID format (.sid) which you can view using some (free) LizardTech software.
You can download the MrSID Viewer (now called ExpressView Browser Plugin (MrSID)) which is an IE plug-in that enables you to view, zoom, etc. The program you want is called “exview_setup.exe”. Once you’ve run it, you then double click on a .sid file and IE will do its new funky stuff for you.
To change the format to, say a JPEG, you will need another piece of (free) LizardTech software called “mrsiddecode.exe”. This an MS-DOS programme, and running on one of these files you may think that your humble little PC has died. Do not worry, go down to the pub for a couple of hours and when you get back you will find that it now nearly 10% complete.