Well, the DNS 'thingie' is the Dynamic Name Server - it translates the text forms of the domain names into the magic IP numbers. The numbers are what the internet uses to find things - the words are hooked to them via lookup tables.
One table may be in your HOSTS file on your machine (strictly, this is checked before asking the net DNS server machines). Some cookies may have the IP numbers embedded in them, and may get looked at first. There may be a routing table kept in your machines RAM - the 'ipconfig /flushdns' will fix this. Some Web accelerators also try tricks like this to keep info stored so as to obviate regular lookups each time the IP number is needed - obviously if that info is corrupt, you will get waylaid.
If the host you are trying to find is changed to use another IP number, that is usually notified to the server machines on the net and propagates around, but if your machine is looking up the IP number from your machine, you are stuffed, until you fix this.
The DSL 'modem' box may have routing tables in it too. These should be transparent and self fixing, but something may not be working right, and should be reset. One would expect that a power on/off would fix this - but it may retain the info internally for a while after power down - how long it needs to be switched off to rest, I don't know. Your DSL ISP should have tools to fix this.
The "If you are using ADSL or DSL make sure that 'dial whenever a network connection is not present' is not enabled. Use 'never dial a connection' instead" hint should not be overlooked too. If you are using both dialup & DSL on the same box, it may have gotten turned on.
The LSP-Fix hint implies that malware may have snuck into the system somewhere - this util does not remove the malware but may repair the broken internal links after the malware has been manually deleted.
"Repairs Winsock 2 settings, caused by buggy or improperly-removed Internet software, that result in loss of Internet access. LSP-Fix is a free utility to repair a specific type of problem associated with certain Internet software. This type of software is known as a Layered Service Provider or LSP, a piece of software that can be inserted into the Windows TCP/IP handler like a link in a chain. However, due to bugs in the LSP software or deletion of the software, this chain can get broken, rendering the user unable to access the Internet. Unfortunately, this type of software is sometimes quietly installed by unrelated software such as file-sharing programs, sneaking onto a system unannounced. In fact, in many cases, the user does not know of its existence until something goes wrong, and he/she can no longer access Web sites."
You shouldn't need a Dos window to run ipconfig - it will work in the Run entry from the start button - if 2000 is like other windows.
Certain AV Sw and other mentioned progs MAy be the culprit - you will have to manually disable them - but if you switch off your firewall to test that - don't stay on the net very long unprotected........