A lot of us are still using XP for various reasons: We don't like Win 8, why pay for an OS when we have one that works and came with the computer, etc.
The short answer, looking at what you want is: No, you can't. Especially the part about "..very secure for along (sic) time".
Every time a new vulnerabilty pops up, Miocrosoft has to scamble to provide patches for all of their systems as quickly as possible. If these stop, you can be very exposed. Software made for XP may stop at some point even from other vendors. We very rarely see software advertised that will run on Windows ME or Windows 98 or even Windows NT anymore. There is still software that could run on Windows 2000, but that is dropping fast.
Although I tend to avoid "free" editions (I still believe in: "you get what you pay for"), you seem to have very good software loaded on your current system. However, I have seen the effects caused by "zero-day" vulnerabities. A small bit of malware that knocked out 100,000 computers in 20 minutes where even the A/V companies didn't know what it was. I feel for your situation. I am on XP myself but I feel that I will probably get a new computer with Windows 7 before the April deadline.
Here is what I would do:
1) Get an external hardware (i.e., expensive) firewall. Keep it very locked-down. I know money seems to be an issue so make sure you have the best firewall you can afford (even if software) and keep it locked down beyond the point of it being a nuisance. The more obnoxious, the better.
2) Avoid online banking or keeping financial information on the computer. However, I don't trust public clouds with this type of info as they all claim to read your stuff. If you do this, make sure you clear your browser's cache, history, etc. after use. Even though your bank may have a "guarantee" that you won't lose money using their site, you still could be subject to identity theft and that takes years and a lot of money to clear up.
3) Plan what software you will need for the time you will be on XP. There may be only versions for newer OS systems out there at some point. So, if you are thinking of using some application in the future, you may need to buy it closer to now rather than later. Also, plan your hardware. If you are planning for a new printer or something that you will buy in the future, be aware that they may not have XP drivers. So, plan accordingly.
4) Browser, XP is limited to, I think, Internet Explorer 8. Maybe 9 but certainly not 10. (OK, I forgot). What happens when websites won't work on old browsers? Yes, there will always be Firefox, chrome, etc. but for how long will these work on XP?
Figure out the true cost of staying on XP with all of the security stuff you are hoping to add and then look at the price for Win 7 or 8 (may not be as expensive as you think). As long as you can get the Dell drivers for your hardware (or Microsoft drivers) off the web, it may be a good move.
I have a Dell Dimension E510 but I suspect that, as long as I have employment, I will eventually get something new. I will just have to plan for it.