You might consider reformatting your computer and re-installing the operating system if it's been bogged down over time by viruses and such. But if you have had these problems from the start, than this is an unlikely fix.
If you are looking for an interface (which would be a very good idea), MOTU is a good bet as DPDan mentioned. MOTU combined audio/MIDI interfaces are certainly not cheap compared to some others, but they would serve you very well. They do make a straight MIDI interface that's more affordable, if you already have an audio interface. There are also reviews on most interfaces out there at Harmony Central:
For the PC vs MAC argument: I personally prefer PC to mac for most work, as I can get right in there and mess around/configure stuff easily, as well as physically build and mod my desktop (fanless water cooling for quiet operation, for instance). But, I do use both. My laptop is a mac, and I enjoy using it when I don't need triple display and a quillian things running at once. If media is your main game, mac is a good choice.
Also, a MAC will typically emit less noise than a store bought PC from what I've seen, as Apple puts some time into industrial design. But oh my does my macbook processor run hot under heavy load, and when it does, the thing makes too much noise for me to comfortably make music/edit on it (this is a core2 duo macbook that's a few years old, not a macbook pro). My DIY built PC barley whispers and never shouts and runs very cool even when safely overclocked, and that is the main reason that I prefer it over mac (the customizability). But this is a very unfair comparison, as I'm comparing a quad core desktop with a dual core macbook.