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You really do need to provide as much information as possible about your environment. Please read Asking Better Questions for some helpful information on how to best ask a question - Asking Better Questions
While we are waiting for that information, you may look at this - Windows 7 Slow Boot
Specifically - Windows 7 Slow Boot
took a machine that was experiencing this exact issue this morning and i set the DNS server settings to statically point to my 2 AD servers and the user was able to log in just fine. As far as my DHCP options for DNS goes, im assigning all PCs my primary DC as the primary DNS and i have a secondary DNS of 8.8.8.8 and a tertiary of 4.2.2.2. Im assuming that for whatever reason, the PCs that are having this issue are jumping over to use the secondary and maybe even tertiary DNS servers but since they are not AD DNS servers, the profile just hangs until it makes it round back to the primary DNS server.
The “why does Windows not always pick the primary DNS server” “fiasco” is coincidently being discussed in this other thread: DNS and small environments …this “feature” has been the way it is for a long time. …my take on it, if I’m not being obvious enough, is you should NEVER specify non-AD-integrated-DNS-servers on end-user devices - because of the chance they will pick another one (2nd, 3rd, 4th, etc.) and then not able to resolve any internal FQDN’s from that point on. …and your internal DNS server(s) should ALWAYS be “given” to the client via DHCP or via Group Policy - setting the DNS server IPs statically can cause you problems if the end-user device visits other networks (especially wireless hotspots or public networks) and this can also be considered a security risk.
As an aside, mixing different DNS providers (8.8.8.8 vs. 4.4.4.2 vs. your ISP vs. something else) under the forwarders tab on your AD-integrated-DNS-server, I don’t like that either - sure, everyone thinks the more you add makes it “more fault tolerant” which I suppose is true, but your also introducing problems with geo-location and global server load balancing. For example 8.8.8.8 will usually be the best bet to accurately route your YouTube traffic to the nearest Google datacenter, making videos stream the fastest, but your ISP, or 4.4.4.2 (who is probably NOT your ISP BTW) won’t necessary make the best choice, and you can be directed to a geographically further datacenter, and thus adversely affect load times. I would test your upstream DNS server using GRC's | DNS Nameserver Performance Benchmark or Google Code Archive - Long-term storage for Google Code Project Hosting. and stick with whatever provider is fastest.