I am going to install Ubuntu to dual boot with Windows 8 soon, but one of my friends warned me that partitioning my hard drive will slow my computer down. I run on a 500GB HDD and 8GB of ram if that helps at all. Is that true if I do it?
There are several factors to take into account but generally speaking, Ubuntu normally is at disadvantage when installing. This is because Windows is normally at the beginning of the disk, and Ubuntu (or any other distro) is at the end. Disks are faster then closer they are to the start, hence Windows will have shortest seek times, that will give it a boost opposed to Ubuntu.
Another thing is that Windows will not run at the same time that Ubuntu, nor the reverse. Hence while you are running Windows, only your system inherent performance is taken into account. I have not seen (nor imagine) any circumstance that Windows will slow down because you installed Ubuntu.
Bottom line, Windows normally will have the advantage on the hardware, but Ubuntu (Linux) will use it more efficiently. That's all.
It would not slow down your machine because you're not firing it up all together :)
The fact that only one operating system will run in a dual-boot setup, hardware resources like CPU and memory is not shared on both Operating Systems (Windows and Linux) therefore making the operating system currently running use the maximum hardware specification. Regarding the disk issue, well unless the windows partition run out of free space which eventually can be fixed by removing some files.
It will slow down your computer in one case: if you have too small amount of free space on your Windows partition. Thus Windows will show drive
D:) with red bar and will prompt you for cleaning. Just remember that you need to keep ~10 GiBs free space on your Windows hard drive and ~40 GiBs for Ubuntu.
While all ten other answers are correct that there will be no noticeable difference in running time. Boot time will plummet on modern PCs as you will not be able to enable bios caching (at least none of my systems will bios cache to grub) and grub will have a pause and select time of it's own which will slow down boot time (configurable).
Bottom line do it but be aware that you can no-longer boot to windows in under 5 seconds (assumes ssd and bios caching)
I would just like to add to this that I have noticed a significant performance decrease in my Windows 7 after repartitioning my Windows drive to create space for Ubuntu, although I did screw around with the partitioning quite a bit (created partition to install Ubuntu to, had some issues, repartitioned, more issues) before actually installing Ubuntu to unpartitioned space.
So I checked on Windows whether the drive might be fragmented because this is the only logical reason that I could think my Windows is running slower after the dual boot installation and only came up with a 1% fragmentation. Still not 100% sure why Windows is running slower now though. I might just re-install Windows to see if that helps (note if anyone else tries this, remember to reset your grub).
Could be that I'm just so used to working on a lightning fast linux machine that Windows feels slow now :P