Right, here's the minimum-faff approach to creating a version of Minecraft to run Technic, separate from your main Minecraft install (that is, the one in appdata/roaming).
1. Download Minecraft.exe, and the Tekkit.7z archive linked earlier.
2. Create a wholly new folder to install Minecraft in, eg. D:\Program Files\Minecraft Technic
3. Save Minecraft.exe into that folder.
4. Create a new notepad file, enter exactly the text:
set APPDATA=http://forums.bethsoft.com/topic/1330349-unofficial-minecraft-thread-70/%cd%minecraft.exe
5. Save the notepad file into the new Minecraft folder above as Technic.bat - Make sure the file type is "All Files".
6. Double click on the newly created Technic.bat. If it opens it up in Notepad again, then you saved it as a .txt. Create it again, and save as "all files".
7. Minecraft will start up. Enter your login details, and allow the game to download all the necessary files to start up.
8. Close the game.
9. Right click the Tekkit archive (Tekkit_1.0.0_Client.7z), click "extract files"
10. In the "Destination path" field, put in the path to your new Minecraft install folder, followed by "\TekkitInstall" (eg, full text entered into the field will be "D:\Program Files\Minecraft Technic\TekkitInstaller")
11. Click "ok".
12. Open up the folder you just created (eg D:\Program Files\Minecraft Technic\TekkitInstaller), double click on Tekkit_Installer (Executable Jar File)
13. Click "set target Minecraft folder".
14. In the Folder Name field, put the path to your new Minecraft install folder (eg D:\Program Files\Minecraft Technic\TekkitInstaller) followed by \.minecraft (eg put D:\Program Files\Minecraft Technic\.minecraft)
15. Click "next".
16. In the "Presets" field, select "full".
17. Click "next".
18. Wait for the installer to run, then click "Done".
19. Right click on your desktop. Select New->Shortcut.
20. Click "browse".
21. Select the Technic.bat file you created earlier (eg in D:\Program Files\Minecraft Technic). Click "ok".
22. Click "next".
23. In the "Type a name for this shortcut" field, type "Minecraft Technic".
24. Click "Finish".
If you follow these 5 minute instructions to the very letter (capitalisations may vary), you WILL have a fully functional version of Minecraft running Technic, which you start via the Minecraft Technic shortcut. You will also retain your old version of Minecraft, which you can play by using your old Minecraft shortcut.