I have a new PC which I've been using for about 9 months with windows XP. However, since of late, the time it takes to load all the startup programs has become quite long.. i.e the time between when I click the login icon and when the system actually becomes usable after loading all the startup programs.
I haven't installed any new software recently, that is, before I noticed an increase in startup time so I don't want to delete any startup programs.
Could it be something like temporary internet files?

I had a slow boot problem as well. I would suggest you first check if you have too many programs loading at boot or if you dont have enough RAM. I was also able to improve boot by running a complete defrag. (Run a diskcleanup before that if you need to)

Hm.. I disabled some startup programs using Msconfig but now every time I start up, I get a message from windows saying some programs have been disabled and that MsConfig is running in some particular mode. How do I fix this (without re-enabling the startup programs)

Hmm.. I gave up trying to use Msconfig. All the startup programs in there were there from the day I got my PC. that is, there are no new startup programs that were installed recently. Therefore, I don't believe it's simple the startup programs that are making start up slow.

use msconfig. start run and write msconfig. then from startup tab unchecked program what you don't need. I didn't face any problem what mahela said. some program start with start windows. so you can disable it.