There are a number of things that come to mind.
1. Network initialization - if Broadband, then DHCP may be
causing slow boot times or if a modem dial-up is used &
PC has a Network Interface card it should be disabled.
2. Startups, Watchdogs and other non-essential services that
programs add to the boot cycle.
3. Spyware & Malware - Cleanup with AdAware/SpyBot &
CWShredder
4. Drivers incorrect or not fully updated.(Video & Chipset)
5. Windows is not fully upgraded with all patches and updates.
6. Programs not at latest version, includes things like Direct-X,
using MS-Java instead of Sun Java.
All of these things should be looked on as similar to an oil change
for your car. PC's most definitely need routine maintenance.