It sounds like a DNS Server issue (or possibly, but less likely, spyware). You may want to change your DNS servers to something else and see if that fixes the problem.
Here's a link on how to do it in Windows XP:
http://www.mediacollege.com/computer/network/dns.html
Here's a list of DNS servers that you could use, you only need to choose two:
Verizon (Level3) Nameservers
4.2.2.1
4.2.2.2
4.2.2.3
4.2.2.4
4.2.2.5
4.2.2.6
SpeakEasy Nameservers
66.93.87.2
216.231.41.2
216.254.95.2
64.81.45.2
64.81.111.2
64.81.127.2
64.81.79.2
64.81.159.2
66.92.64.2
66.92.224.2
66.92.159.2
64.81.79.2
64.81.159.2
64.81.127.2
64.81.45.2
216.27.175.2
66.92.159.2
66.93.87.2
ORSC Public Access DNS Nameservers
199.166.24.253
199.166.27.253
199.166.28.10
199.166.29.3
199.166.31.3
195.117.6.25
204.57.55.100
Sprintlink General DNS
204.117.214.10
199.2.252.10
204.97.212.10
Cisco
128.107.241.185
192.135.250.69