Safari Problems, occasionally quits out

I have had a few problems with my mac, mainly leopard issue when I upgraded, so decided to re-install tiger, so mac is up and running good again, the trouble is that safari occasionally quits out, and very rarely word quits or just will not run, , , , any ideas.

Running safari 3.0.4
Mac OSX Version 10.4.11
2.16 GHz Intel Core Duo
1.5 GB 667 MHz DDR2 SDRAM

To all you experts, how does a web site know what software you are running on your computer so it knows what upgrade/patch etc to use, especially if you automatically upgrade.
Thanks.

I did a clean install, erased disk from utilities etc , , , , , , most things are running fine just the two problems with safari and word.