The Mydoom.B virus will overwrite the systems hosts file as per the following:

Overwrites the local host file to prevent users from accessing the following sites. This action occurs at each instance the worm runs. If the local system date falls within the date range of the www.microsoft.com DoS, the reference to www.microsoft.com is not written to the hosts file.

ad.doubleclick.net
ad.fastclick.net
ads.fastclick.net
ar.atwola.com
atdmt.com
avp.ch
avp.com
avp.ru
awaps.net
banner.fastclick.net
banners.fastclick.net
ca.com
click.atdmt.com
clicks.atdmt.com
dispatch.mcafee.com
download.mcafee.com
download.microsoft.com
downloads.microsoft.com
engine.awaps.net
fastclick.net
f-secure.com
ftp.f-secure.com
ftp.sophos.com
go.microsoft.com
liveupdate.symantec.com
mast.mcafee.com
mcafee.com
media.fastclick.net
msdn.microsoft.com
my-etrust.com
nai.com
networkassociates.com
office.microsoft.com
phx.corporate-ir.net
secure.nai.com
securityresponse.symantec.com
service1.symantec.com
sophos.com
spd.atdmt.com
support.microsoft.com
symantec.com
update.symantec.com
updates.symantec.com
us.mcafee.com
vil.nai.com
viruslist.ru
windowsupdate.microsoft.com
www.avp.ch
www.avp.com
www.avp.ru
www.awaps.net
www.ca.com
www.fastclick.net
www.f-secure.com
www.kaspersky.ru
www.mcafee.com
www.microsoft.com
www.my-etrust.com
www.nai.com
www.networkassociates.com
www.sophos.com
www.symantec.com
www.trendmicro.com
www.viruslist.ru
www3.ca.com
This will result in you not being able to contact any of the listed sites. You may want to check to see if your hosts file has been tampered with.

Cheers: