Is it more likely to be the tuners or the nut? I have a crappy fixed bridge strat, i have moded it up nice, but it still just goes out of tune all the time.

Any advice would be great, thanks
RG's & Mesa's
Mostly, it's just the strings stretching and moving as you play. If you do bends and any degree of vibrato, your strings are gonna gradually go out of tune, if you use light gauge strings, then they're gonna go out of tune quickly.
The tuners on the cheap guitars are pretty much junk. Its the reason the guitar was cheap. A set of no name "economy" tuners are 10 bucks even less in bulk. A set of grovers are 35 bucks. So on a 100 dollar guitar be pretty hard to put good tuners and still make any money.
It's a combination of the tuners and the nut most likely. Bad tuners are just that, bad tuners, but the reason why I mention the nut, is because if you do a lot of bending or vibrato, it's possible that the string could be binding (getting stuck) at the nut instead of sliding back to it's original position. You'll notice it sometimes if you're tuning, then suddenly hear a ping and your string goes sharp. The string gets stuck in the nut as your tuning, then finally skips free, making that pinging sound.

Either Big Bends Nut Sauce, Graphitall, or simple pencil lead dust can help alleviate that. Or replacing the nut with something like a Graphtech nut.

Also, another major factor in tuning is how you string your guitar. If your strings aren't locked properly at the machine heads, slippage could occur, causing the strings to go flat over time. I use the following method to string my guitars and personally, it's worked great.

~We Rock Out With Our Cocks Out!: UG Naked Club.~
Once in a blue moon, God reaches down from his lofty perch, points at an infant boy and proclaims, "This one shall have balls carved out of fucking granite."
Quote by XianXiuHong
Usually, its the tuners, otherwise players world-wide would buy Squiers and replace the nut.

ha ha xing xong, there is more to a guitar than the tuners unfortunately
RG's & Mesa's
Actually, you know what it is the vast, vast majority of the time?

It's caused by not re-stringing it properly.

I have never found a single guitar - from Custom Shop models worth thousands right down to £60 plywood crap - that can't hold tune, so long as it was actually re-strung currectly. If you're having tuning problems, the first thing you should do, is re-string it again, and make sure you're doing it properly. If you're not sure if you're doing it properly or not, take it to a store and ask them to do it (might as well get a full set-up while you're at it). In 99.9999% of situations, this will solve all your tuning problems.