Divx requires a special codec not included with XP, you'll need to visit a
Divx site for the appropriate player and/or codec.
As to AVI files, I believe Windows Media Player will play them but you may
need the proper codec. If a special codec is necessary, usually the person
supplying the file supplies the proper codec so you'll have to ask the
source.
An AVI file can be compressed or uncompressed. Since an uncompressed AVI
file is about 12gb per hour of 'footage' most files you will download will
be compressed. Windows Media Player will happily play an uncompressed AVI
file with no extra codecs being necessary.
Compression can be achieved with different codecs, the most common being
DIVX and xVID. To play the compressed AVI file, you need to have the
correct codecs installed on your PC.