When I start xp my DSL connection no longer connects automatically. My first workaround was to disable and re-enable the LAC. I now restart the DHCP client which seems to work too. Event viewer shows DHCP related errors. I know neither what's broken nor how to fix it.
Whar I do know is that it's strictly an XP problem - I triple boot and have no issues with either Mint or Vista. I've re-installed the ethernet card driver and reset/dumped TCP/IP.
Does anyone have any ideas or suggestions?

Thanks in advance.

Sounds to me like you may have multiple DHCP servers. If you have more than one trying to contorl your IP's you will likely run into problems.
multiple dhcp - Google Search

So, how can I repair this? How to check for DHCP servers? Any advice you can give?

how many internet connections show up in xp. Remove all of them and reinstall just the one set up and see if that doesn't clear it up.

If it is only one make sure the name of the connection is not numbered if it is change the name!.

There is only one LAC, the wireless connection is disabled and I can't figure out how to remove it - it says that this connection cannot be removed. The LAC is named just that - LAC.

look into the router settings and see what all is in there for the DHCP server connections....You might want to reset everything back to factory defaults.