Here are a few things to do:
1. Only use one anti virus program at a time. Only one is necessary, and it takes a long time for even one of them to start up when you log in.
2. Upgrade to 2GBs of memory. 1GB is a little small for Home Premium. Since you can probably do this for less than $20, it's a no-brainer.
3. Defragment your hard disk once a week. You can set the defragment program to automatically do this.
4. Eliminate any startup programs that you don't use. Have the computer only start up the programs that are absolutely required, like your virus software. Excess startup programs are usually the cause of slow startup.
5. Try to shut the computer off as little as possible. Use sleep mode instead.
6. Upgrade to Windows 7 a few months after it's released. I tried the trial version, and it seemed to run a good deal better than Vista. But, wait a few months so they can work out all the bugs and problems that will surely arise as soon as it's released to the public.
Thats about all the tricks I know. I have an older Dell with Vista Home Premium. It now has 3GBs of memory, cheap dual core AMD processor, and a 5400 RPM hard drive. With those specs, it's about maxed out, but it still takes about 2 minutes to start up.
Like I said above, the virus software is probably one of the things slowing the computer down the most. I have just one program (Live OneCare), and it takes about 45 seconds to start it up. Besides, you really only need one program.
I really doubt that a virus or malicious program is your problem. It isn't uncommon for a Vista computer to take 2-6 minutes to startup, because computer manufacturers put the operating system in computers that cannot run it very well in order to make cheaper machines. It's just the way it is, and you can't do much much about it, other than buy a really fancy, expensive, computer.
Try these things out, and you'll probably be able to get it down to 2 minutes or less.