Ad-Aware shouldn't slow down your computer.
You haven't said what application you use to open emails. Outlook? Outlook Express? Yahoo Mail? Hotmail? Your ISP's web mail client?
Anyway, you probably still have a virus or malware that Ad-Aware on its own won't get rid of, so here goes:
Download and install Mozilla Firefox
Using Firefox as your browser, download Spybot Search & Destroy, SUPER AntiSpyware, and Spyware Blaster. They are all free and you can get them from www.download.com, www.pcworld.com, or www.tucows.com. Download them ONLY - don't install them right away. Boot to safe mode and install and run them from there.
Also, using Firefox, download AVG Free and Avast free edition. Install and run them in safe mode, but it's probably best to install one of them, run it, then un-install it, before proceeding with the other one.
Once you've done all this, re-boot into normal mode, and run everything again (get any ant-virus and spyware updates while you're in normal mode).
Then, run Windows Update or Microsoft Update, but don't accept the update for IE7 if prompted.
Run Windows Defender.
Download and install WinPatrol Free. It prevents hijackings (un-authorized changes to your IE settings).
You should also check Apllications and Processes to make sure that no un-needed applications are running. Press CTRL+ALT+DEL (all at the same time) and click on the Applications tab. Stop any apps that you don't want by highlighting them and clicking on "End Task". Then click on the Processes tab. Be careful with this, but you can also stop any unwanted processes by highlighting them (one at a time) and clicking on "End Process". This won't delete the process, it just stops it for this session. The worst that can happen is something that you need will stop working so just re-boot.
You can also got to your Start Up and remove any apps from there that you don't always need on start up. Go to Start, Run, type msconfig in the box and hit OK. Open the Startup tab and uncheck and programs (apps) that you don't need on start up (like Adobe Updater, Quicktime, etc.). Click on OK and then re-boot. When it starts up it will warn you that you have made a configuration change but just click on OK or Accept or whatever it says and then proceed as normal.