Any DNS resolution problems would most likely lie with your ISP. Each ISP maintains its own DNS servers for its customer's use. The ISP's servers query the root servers and authoritative DNS servers for the respective domains and cache the results until they expire at which time they are requeried.
If you have problems, open a DOS COMMAND window, type nslookup and then type the domain name you are searching for. this query will be made against your ISP's servers. Then go to
http://dnsstuff.com and query for the same domain name. If dnsstuff returns results and nslookup does not, the problem is with your ISP. If neither returns results, then there is probably a problem with the authoritative servers for that domain.