I'm afraid that if the hotmail doesn't appear on ebay or in your ebay profile then it could be entirely co-incidental.
Where else do you use your hotmail address, usenet newsgroups for example ?
Who do you give your hotmail address to ?
Their system could be infected and sending out spam to all addresses in the addressbook.
Your best defence to unauthorised access to your system apart from what Tech suggests is a good firewall (not XP's firewall). Any malware that manages to get past your defences will have free reign to connect to the internet to either download more of the same, pass your personal data (user names, passwords, keylogger retrieved data, etc.) or open a backdoor to your computer, so outbound protection is essential.
Zone Alarm free
http://www.zonelabs.com works fine with avast and has a reasonably friendly user interface. There are others, Jetico, Sunbelt Kerio, etc.
See some firewall tests for comparison, some are freeware but many are paid for versions
http://www.firewallleaktester.com/tests.php. Also see
http://www.thefreecountry.com/security/firewalls.shtml