First off what version of Windows are you running? If it's Windows XP Pro, Vista Business or Ultimate then you'll have Remote Desktop built in, which will allow you to remotely connect to your PC, as long as your PC is switched on and your router has been configured to allow port 3389 traffic to be passed to your PC.
If you don't have Remote Desktop look at VNC, works in a similar way as long as it's configured correctly.
As for the apps for the iPhone there a few to choose from I personally use Jaadu Remote Desktop, very well written but not the cheapest. There are free lite version remote control apps for both Remote Desktop & VNC on the app store they won't offer as many features as the paid versions but they do the job.
Could try logmein via the web onto your machine?
Using Safari...
Doesn't work (well it certainly doesn't for me).
I've found the iPhone screen a bit small for effective remote desktop in practice.
Can't use logmein via the standard web interface as it needs to install components that Safari on the iPhone won't support.
They do do an iPhone client but it doesn't work with their free product unfortunately.
I use the free VNC client with my macbook pro as VNC is installed as part of osx, however I don't do anything important with it.
what is it you wish to do remotely as its a bit of a pain viewing it on a screen so small.
iRdesktop is another free one (basic but does the job)
The main reason at the time for wanting to use my PC remotely was to start the new BF2 1.5 patch download.
I was at work and wanted it downloaded before I got home.
yer its very useful for stuff like this as i said give vnc a go.