** This issue is addressed in ArcGIS 9.0 Service Pack 2 (SP2) **
Connecting to an ArcIMS Service in ArcMap through a proxy server causes these applications to crash.
This is a known issue.
A crash occurs when the proxy server information is present in both ArcCatalog [Tools > Options > Proxy Server] and the Internet Explorer connections [Tools > Internet Options > Connections > LAN Settings].
** This issue is addressed in ArcGIS 9.0 Service Pack 2 (SP2).
Click on the link in Related Information below and download Service Pack 2, if necessary. **
If the Service Pack 2 is unavailable, disable the proxy server in the Internet Explorer connection.
Alternatively, try adding the HTTP site to the exceptions list in Internet Explorer. That is, open the 'Proxy Settings' dialog in Internet Explorer [Tools > Internet Options > Connections > LAN Settings > Advanced], and enter the HTTP site in the Exceptions field.