I've no idea why it should suddenly start to boot slowly, and I'll leave the possible networking issues aside as I'm not a networking whizz, but you could try a couple of things to look at the lap top itself (worth a try at least).
Firstly, have you tried msconfig? If not, open up a run box and type msconfig into it. Click on the start up tab and check to see if anything has crept in their that maybe shouldn't have.
Secondly, again not a miracle cure I suspect but worth a try at least, pay a visit to
http://www.blackviper.com/WinXP/servicecfg.htm and see if you can cut down on the number of drivers and services that are loading with XP. There is a safe option which I'd recommend and, as you're planning on using the networking features I wouldn't disable any of the associated services without remembering how to re-enable them should anything go wrong.
I've also found on the web the following advice to remedy what is a known bug with XP networking, and it only seems to pop up every once in a while. Apparently mrxsmb.sys doesn't always load correctly, leading to very very long boot times. The fix is simple - disable and then re-enable file and printer sharing as follows (apologies if I'm teaching you to suck eggs here by the way) :
1. Alt-click (or right click) on Network Places > Properties
2. Alt-click on Ethernet Adaptor connection > properties
3. Un-check "File and Printer Sharing for Microsoft Networks" > OK
4. reboot
5. If you need file or printer sharing, repeat the above, re-check the box and re-boot again.
If all of this fails download a little app called "BootVis" and that'll show you precisely what's taking up your boot time. It's produced by Microsoft and you can get it directly here :
http://www.mvps.org/sramesh2k/utils/BootVis.exe
Hope all of this waffle helps
