Can anyone explain...
 

Can anyone explain the meaning of the values in /proc/loadavg
and the disk_io line in /proc/stat. Also, is there an easy way to see when was the last time the computer was booted?

For the "/proc/loadavg",
The first three columns measure CPU utilization of the last 1, 5, and 10 minute periods. The fourth column shows the number of currently running processes and the total number of processes. The last column displays the last process ID used.

For the "disk_io"in "/proc/stat", the format is,
(major,minor) : (read+write-requests,read-requests,read-sectors,write-requests,write-sectors)
(I don't know other than this)

You can calculate the last bootime from "/proc/stat" file "btime" field.
btime — The boot time, measured in the number of seconds since January 1, 1970.