I use ESET SS which includes a firewall and a AV that updates around 6+ times a day. Any firewall will work as long as it has a interactive mode, which means you select what you want to run. The drawback is that some items may look fishy in some way, but their actual needed by your system.
If you're on windows then there are a few remote desktop services that are able to run in the background. I would say you would never need these depending on your situation. You can knock a few out in your control panel>network sharing center. You can configure things from there just allowing no sharing or any access to your network. This may not work if she has a router which needs to detect PCs on the network, since sometimes ISPs change IPs after some redials.
The other users pointed out a lot of good information on programs to protect/detect. One that really comes to mind is HiJackThis. It's a free application which you can download from a freeware site. This can detect a lot of apps and registration files. If you do get it and scan your PC, you can save the log (completed list) or click analyzeThis which tells information about a select item. I wouldn't use the analyzeThis option though.
Some programs may be detected, but may not have any information about them. For that case I would join a PC forum, any PC forum basically and make a post showing your log file. Most of the users can pinPoint any files that look fishy, etc.
I use Firefox and it's not secure without addons. You may have ran into one of those, this site is blocked type scenarios. Well that's the only useful function it has, which is default. I can't comment on any other browser.
Firefox has a save password option which can be obtained through it's appdata folder I think. It my be encrypted though, but it wont stop anyone who wants your shit. The passwords are also visible through Tools:Options:Security:Save Passwords. For this type of thing, there is an addon called noscript and a useful one called AdBlockPro. It's a start, but wont save anyone from getting into that file. Their just for blocking flagged sites or scripts that may run in the background of a website (IE: iframes). You can resort to completely not allowing anything to be saved through firefox.
I basically do all the above and stay pretty clean. The only problem I ever encountered was due to Firefox, which I detected a flagged javascript file.
You can surf on a Virtual Windows Machine, but I would just get it right the first time. Not sure if security is a daily thing or you just need it for certain occasions, but the VM remark that cyber pointed out is a way to go. There is always the proxy option, but their slow and SOME proxies are actually supplied by certain IT companies within the government. I wont go into detail on that, but a lot of people get canned.
Another thing, if you download something from somewhere/someone. Scan the file with virustotal(dot)com. It scans with around 20 or so programs and may flag a fishy item. There is no bulkUpload option so if you're curious about multiple items then place them in a folder and zip/rar it (winzip/winrar, both freeware). The file can't be over 20mb I think. If you scan it that way and it pulls up a alert, then scan them one by one until you find the culprit. Some alerts may be "falseflag", do a google to learn more on that, it doesn't mean their bad.
Yeah it's probably confusing, but thought I would get that out there incase anyone was also browsing and wanted to know.
EDIT: Another thing on USBs. You shouldn't worry about them unless you use it as a constant memory source. Not sure how you can protect that aside from basic computer security. Except allowing an outside source to access the data from it. If you carry it around, you can password lock files with certain apps or you can use a software which autoboots a program on it to password protect it. USB Secure does that, along with what some users above mentioned.
Big brother snooping software - how to detect?
