If you said that you performed scans for malware and reinstalled the application then the only problem consists in the configuration of your anvitirus/antispyware/firewall application. Often these applications consider Internet Explorer to be a kind of threat based on the websites you visited.
In Vista open a cmd prompt (using RUN from Start Menu) with elevated privileges and type: netsh advfirewall. Check there and see if your firewall is blocking Internet Explorer and also check your anti-malware solutions by adding Internet Explorer to the Exception list. Slow loading of webpages may be caused by aggressive filtering.
