a. Fetch your certificate

1.You will receive an email when your certificate is issued.Download your certificate and Copy and Paste your thawte certificate to Notepad and save as a cert.cer

b. Install your certificate

1. Open Internet Services Manager, or the custom MMC containing the Internet Information Services snap-in.
2. Expand Internet Information Services (if needed) and browse to the Web site you have a pending certificate request on.
3. Right-click on the site and then click Properties.
4. Click the Directory Security tab.
5. Under the Secure Communications section, click Server Certificate.
6. On the Web Site Certificate Wizard , click Next.
7. Choose to Process the Pending Request and Install the Certificate. Click Next.
8. Type in the location of the certificate response file (you may also browse to the file) , and then click Next.
9. Read the summary screen to be sure that you are processing the correct certificate, and then click Next.
10. You will see a confirmation screen. When you have read this information, click Next.
11. Right click the web -site to go into properties. From there click the web-site tab.
12. Make sure that the secure port 443 is enabled and an IP address is assigned.

Follow the instructions at the following link to enable SSL on IIS 5.0 : http://support.microsoft.com/view/tn.asp?kb=298805

You now have a server certificate installed. You may want to test the Web site to ensure that everything is working correctly. Be sure to use https:// when you test connectivity to the site
Open the ISA Manager and complete the SSL installation:
1. Right-click the server that is going to accept the incoming connection, and then click Properties.
2. Click the Incoming Web Requests tab.
3. Click the Internet Protocol (IP) address entry for the site that you are going to host, or the all IP addresses entry if you do not have individual IP addresses set up.
4. Click Edit.
5. Click to select the Use a server certificate to authenticate to web users check box.
6. Click Select.
7. Select your previously imported certificate.
8. Click OK.
9. Click to select the Enable SSL listeners check box.
10. Expand the Publishing folder, and then click Web Publishing Rules.
11. Double-click the Web publishing rule that will route the SSL traffic.
12. On the Bridging tab, locate Redirect SSL requests as, and then select HTTP requests (terminate the secure channel at the proxy) .
13. Click OK.
Restart ISA Server.

For more information please refer to this Microsoft solution: http://support.microsoft.com/default.aspx?scid=kb;en-us;324167