First check ipaddress
Click Start -run- type cmd hit enter for XP or Start -search-type cmd hit enter in Vista. On the DOS black screen after the blinking cursor type ipconfig hit enter check the ipaddress if are getting ipaddress other than (169........) it is proper ipaddress for e.g 147.14.48.478
Ping the default gateway e.g ping 147.11.445.478 if you are able to get packets send 4 received 4 loss zero is with Internet explorer or operating system.
If unable to ping or getting 169..... ipaddress . you are having static ip address
On the black screen at the blinking cursor type ipconfig /release hit enter
then type ipconfig /renew hit enter .check ip address
Ping default gateway then ping
www.google.com if you are able to get packets send 4 received 4 loss 0. Try connect to internet using internet explorer or other browser. if still unable to connect then issue with firewall or antivirus or IE corruption .
Create new administrative account in XP and do
MSconfig .Restart the system and logon to new account and try connecting to internet.
If issue persist might have to repair IE or operating system as it is software issue or virus issue.
You can connect a different computer if you have one and check internet.
If unable to get any ipaddress or still getting static ip address ,reset modem or check ethernet cable ,do a hard reset on the internet modem.Shutdown and restart computer and check ipaddress then try connecting to internet. Good luck.